is light travel in straight line

Does Light Travel in a Straight Line? Can It Be Bent?

Last Updated on Jan 27 2023

A basic principle of physics states that light travels in a straight line. It’s easy to prove, too. Simply shine a light through a parallel series of openings and it will pass through each one successively. You can also see it in real-time when looking at shadows. The division between the lighted area in the background and the object obstructing the light follows the perimeter. 

Keep reading to learn more about how light travels and more pertinent information regarding the subject.

• Light vs. Sound

We can learn more about the properties of light by comparing it to another intangible force: sound. Sound is variable, depending on what it’s traveling through it and its temperature. At 59℉ at sea level, it will go 761.2 miles per h our (mph) . That may sound fast until you start delving into Albert Einstein’s Theory of Special Relativity . Essentially, nothing can exceed the speed of light.

While sound is moving around at 761.2 mph, light is zipping along at a blazing 983,571,056 feet or 186,282 miles per second. It’s worth noting that this measurement is in a vacuum. Light sound, the medium—in this case, air—can affect its speed.

We can put the two properties in perspective with lightning and thunder. Remember that that bolt is moving 186,282 miles per second. The thunder is lagging behind at 1,100 feet per second . Both are moving at a constant speed, making it easy to calculate the distance between the lightning and the clap of thunder. Count the seconds between the two and divide by five to get the number of miles away.

If you’ve seen lightning strike, you’ll notice it’s following a straight path, although it may come at an angle. However, does that mean that light never deviates from this course? The answer is no.

• Scattering the Light

If you’ve seen light shine through a cloud of dust, you may notice that it’s traveling in different directions. That’s the variations in the air medium changing with the suspended particles. Another classic example involves putting an object like a spoon in a glass of water. It will look like it’s bent. What you’re seeing is the difference between traveling through air and water.

Air is composed primarily of nitrogen, oxygen, water, and carbon dioxide. Of course, water is hydrogen and oxygen. However, there’s also the glass, which adds another factor to the mix. A prism will have a similar effect by refracting light into its various colors. However, we still have to dig a bit deeper. All things being equal, can light ever bend on its own?

• Bending Light

Scientists thought that they had solved these riddles until they discovered the Airy waveform in the late 1970s. Researchers found that light could bend ever so slightly. Next, fast forward to 2012. The reason behind the discovery is based on heavy-duty mathematics and physics . Suffice to say that self-bending light is possible, opening up opportunities to use it for various purposes, such as redirecting lasers.

It might not be something that you would ever need to do. However, it does answer some questions. So, yes, light can travel in a straight line and also bend.

• Final Thoughts

Understanding how light and sound travel tells us a lot about physics, mathematics, and science. It also shows us how much we have yet to learn about our planet and its place in the Solar System . While light does travel in a straight line, there are also times when it can bend.

• https://www.livescience.com/37022-speed-of-sound-mach-1.html
• https://www.space.com/36273-theory-special-relativity.html
• https://www.space.com/15830-light-speed.html
• https://www.grc.nasa.gov/www/k-12/airplane/sound2.html
• http://tornado.sfsu.edu/geosciences/classes/m201/Atmosphere/AtmosphericComposition.html
• https://www.britannica.com/technology/prism-optics
• https://www.science.org/content/article/light-bends-itself
• https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.108.163901

Featured Image Credit: donatas1205, Shutterstock

Table of Contents

About the Author Chris Dinesen Rogers

Chris has been writing since 2009 on a variety of topics. Her motto with all of her writing is “science-based writing nurtured by education and critical thinking.” Chris specializes in science topics and has a special love for health and environmental topics, and animals of all shapes and sizes.

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NOTIFICATIONS

Light basics.

• + Create new collection

Light is a form of energy produced by a light source. Light is made of photons that travel very fast. Photons of light behave like both waves and particles.

Light sources

Something that produces light is called a light source. There are two main kinds of light sources:

Fireworks show how light travels faster than sound. We see the light almost instantly, but the sound arrives later. To work out how many kilometres away the fireworks are, count the seconds until you hear the bang and divide by 3.

Incandescent sources use heat to produce light. Nearly all solids, liquids and gases will start to glow with a dull red colour once they reach a temperature of about 525 °C. At about 2300 °C, the filament in a light bulb will start to produce all of the colours of the visible spectrum, so it will look white. The Sun, stars, a flame and molten metal are all incandescent.

Luminescent sources are normally cooler and can be produced by chemical reactions, such as in a glowstick or a glow-worm. Other luminescent sources include a computer screen, fluorescent lights and LEDs.

Light travels much faster than sound

Light travels at a speed of 299,792,458 m/s (that’s nearly 300,000 km/s!). The distance around the Earth is 40,000 km, so in 1 second, light could travel seven and a half times around the world.

Sound only travels at about 330 m/s through the air, so light is nearly a million times faster than sound.

If lightning flashes 1 kilometre away from you, the light reaches you in 3 millionths of a second, which is almost instantly. The sound of the thunder takes 3 seconds to travel 1 kilometre – to work out many kilometres away lightning is, count the seconds for the thunder to arrive and divide by 3.

Lightning storms are important for converting nitrogen gas in the atmosphere through to forms that are biologically available.

Light takes about 8 minutes and 20 seconds to reach the Earth from the Sun. When we see the Sun, we are seeing what it looked like over 8 minutes ago.

Light can travel through empty space

Unlike sound, which needs a medium (like air or water) to travel through, light can travel in the vacuum of space.

Light travels in straight lines

Once light has been produced, it will keep travelling in a straight line until it hits something else.

Shadows are evidence of light travelling in straight lines. An object blocks light so that it can’t reach the surface where we see the shadow. Light fills up all of the space before it hits the object, but the whole region between the object and the surface is in shadow. Shadows don’t appear totally dark because there is still some light reaching the surface that has been reflected off other objects.

Once light has hit another surface or particles, it is then absorbed, reflected (bounces off), scattered (bounces off in all directions), refracted (direction and speed changes) or transmitted (passes straight through).

Models for light

Wave length, height and frequency

A wave can be described by its length, height (amplitude) and frequency.

Light as waves

Rainbows and prisms can split white light up into different colours. Experiments can be used to show that each of these colours has a different wavelength.

When white light shines through a prism, each colour refracts at a slightly different angle. Violet light refracts slightly more than red light. A prism can be used to show the seven colours of the spectrum that make up white light.

At the beach, the wavelength of water waves might be measured in metres, but the wavelength of light is measured in nanometres – 10 -9 (0.000,000,001) of a metre. Red light has a wavelength of nearly 700 nm (that’s 7 ten-thousandths of a millimetre) while violet light is only 400 nm (4 ten-thousandths of a millimetre).

Visible light is only a very small part of the electromagnetic spectrum – it’s just that this is the range of wavelengths our eyes can detect.

Light as particles

In 1905, Albert Einstein proposed that light is made of billions of small packets of energy that we now call photons. These photons have no mass, but each photon has a specific amount of energy that depends on its frequency (number of vibrations per second). Each photon still has a wavelength. Shorter wavelength photons have more energy.

The photoelectric effect

University of Waikato science researcher Dr Adrian Dorrington explains the photoelectric effect. He then describes how camera sensors can be designed on the basis of this effect to enable light energy to be converted into electric potential energy.

The photoelectric effect is when light can cause electrons to jump out of a metal. These experiments confirm that light is made of these massless particles called photons.

Simple explanations of some of these concepts can be found in the article Building Science Concepts: Shadows .

Nature of science

In order to understand the world we live in, scientists often use models. Sometimes, several models are needed to explain the properties and behaviours of a phenomenon. For example, to understand the behaviour of light, two models are needed. Light needs to be thought of as both waves and particles.

Useful links

Even though light doesn’t have mass, learn how it still has a tiny amount of momentum. Find out about NASA’s solar sails to power spacecraft.

Read about the LightSail project, a crowdfunded project from The Planetary Society, aiming to demonstrate that solar sailing is a viable means of propulsion for CubeSats (miniature satellites intended for low Earth orbit).

Explore solar sails more in your classroom, with this activity Solar Sails: The Future of Space Travel from the TeachEngineering website.

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Why Does Light Travel in a Straight Line?

Most recent answer: 11/20/2010

(published on 11/20/2010)

Follow-Up #1: gravity bending light

Yes. Light from a torch is just like any other light. Astronomers routinely see evidence of light being bent by gravity.

(published on 03/08/2018)

Follow-up on this answer

Related Questions

• magnetism and gravity in the cosmos
• Questions about nebulae
• How do we know that half of the Galaxies are not Anti-matter?
• Bending light with gravity
• How do we know a galaxy or star is 13 billion light years away?
• Is cosmic microwave background from old galaxies?
• Is there a danger of a GRB striking the earth or sun?
• Rotation of galaxies
• Do we feel acceleration in an accelerating universe?
• Can stars exist independently of galaxies ?

Still Curious?

Expore Q&As in related categories

• The Rest of the Universe

• Light Travels In a Straight Line

Light travels in a straight line can be observed by keeping an object in the path of light. In an atmosphere which is bit dusty, we can see light traveling in a straight line. Light emerging from the torch, train and lamps always travel in a straight line. Let us study in detail how does light travel in a straight line.

Suggested Videos

Light travels along a straight line.

Life without light would have been pretty dull. Light travels at a speed of 186,000 miles per second. You must have observed that in your house that whenever a beam of light enters a dark room through a tiny hole in the window, the lightwave always travels in a straight line.

Let us carry out a small activity to show that lightwave travels along a straight line. Take three CD’s and align them together. Align them in such a way that all the CD’s line in a straight line. Now take a candle and place it at the other end. Do make sure that the tip of the candle and the holes of the CD’s all lie in the straight line. Ensure that the height of the CD’s and the tip of the candle are same. Observe the flame of the candle. We are able to see the flame of the candle because the light wave travels through the holes and reaches our eye.

Browse more Topics under Light

• Reflection of Light
• Sunlight – White or Coloured
• Images Formed By Lenses

Now if suppose we displace the center of the CD’s we observe that we are not able to see the flame of the candle. Why does that happen? This is because the light gets blocked. If the light could have the ability to take a curve and travel, we could have seen the lightwave. But since light travels in a straight line, we were unable to see the flame of the candle when the CD is displaced. This proves that light travels along a straight line.

(Source: Wikipedia)

In the above picture, we can clearly see that light coming through the holes in the window travel along a straight line.

Questions For You

Q1. The phenomenon in which the moon’s shadow falls on earth,  or the earth casts its shadow on the moon, is known as

• Lateral deviation

Answer: C. The phenomenon in which the moon’s shadow falls on earth or the earth casts its shadow on the moon is known as an eclipse. During a solar eclipse, moon’s shadow falls on the earth. During a lunar eclipse, earth’s shadow falls on the moon.

Q2. Two examples of non-luminous objects are

• Stars and Moon
• Burning candle, glowing bulb
• The moon, a spoon
• Stars, a spoon

Answer: C. Non-luminous objects are those that do not emit light. The moon and the spoon do not emit light. So these two are good examples of non-luminous objects.

Q3. We can see the objects only when

• Reflected light from the object reaches our eye.
• The objects absorb all the light.
• When the objects allow all the light to pass through them.
• None of these.

Answer: A. Objects can only be seen when light falls on the object and are reflected back to our eyes.

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May 20, 2016

How does light travel?

by Matt Williams, Universe Today

Ever since Democritus – a Greek philosopher who lived between the 5th and 4th century's BCE – argued that all of existence was made up of tiny indivisible atoms, scientists have been speculating as to the true nature of light. Whereas scientists ventured back and forth between the notion that light was a particle or a wave until the modern, the 20th century led to breakthroughs that showed that it behaves as both.

These included the discovery of the electron, the development of quantum theory, and Einstein's Theory of Relativity. However, there remains many fascinating and unanswered questions when it comes to light, many of which arise from its dual nature. For instance, how is it that light can be apparently without mass, but still behave as a particle? And how can it behave like a wave and pass through a vacuum, when all other waves require a medium to propagate?

Theory of Light in the 19th Century:

During the Scientific Revolution, scientists began moving away from Aristotelian scientific theories that had been seen as accepted canon for centuries. This included rejecting Aristotle's theory of light, which viewed it as being a disturbance in the air (one of his four "elements" that composed matter), and embracing the more mechanistic view that light was composed of indivisible atoms.

In many ways, this theory had been previewed by atomists of Classical Antiquity – such as Democritus and Lucretius – both of whom viewed light as a unit of matter given off by the sun. By the 17th century, several scientists emerged who accepted this view, stating that light was made up of discrete particles (or "corpuscles"). This included Pierre Gassendi, a contemporary of René Descartes, Thomas Hobbes, Robert Boyle, and most famously, Sir Isaac Newton.

Newton's corpuscular theory was an elaboration of his view of reality as an interaction of material points through forces. This theory would remain the accepted scientific view for more than 100 years, the principles of which were explained in his 1704 treatise "Opticks, or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light". According to Newton, the principles of light could be summed as follows:

• Every source of light emits large numbers of tiny particles known as corpuscles in a medium surrounding the source.
• These corpuscles are perfectly elastic, rigid, and weightless.

This represented a challenge to "wave theory", which had been advocated by 17th century Dutch astronomer Christiaan Huygens. . These theories were first communicated in 1678 to the Paris Academy of Sciences and were published in 1690 in his "Traité de la lumière" ("Treatise on Light"). In it, he argued a revised version of Descartes views, in which the speed of light is infinite and propagated by means of spherical waves emitted along the wave front.

Double-Slit Experiment:

By the early 19th century, scientists began to break with corpuscular theory. This was due in part to the fact that corpuscular theory failed to adequately explain the diffraction, interference and polarization of light, but was also because of various experiments that seemed to confirm the still-competing view that light behaved as a wave.

The most famous of these was arguably the Double-Slit Experiment, which was originally conducted by English polymath Thomas Young in 1801 (though Sir Isaac Newton is believed to have conducted something similar in his own time). In Young's version of the experiment, he used a slip of paper with slits cut into it, and then pointed a light source at them to measure how light passed through it.

According to classical (i.e. Newtonian) particle theory, the results of the experiment should have corresponded to the slits, the impacts on the screen appearing in two vertical lines. Instead, the results showed that the coherent beams of light were interfering, creating a pattern of bright and dark bands on the screen. This contradicted classical particle theory, in which particles do not interfere with each other, but merely collide.

The only possible explanation for this pattern of interference was that the light beams were in fact behaving as waves. Thus, this experiment dispelled the notion that light consisted of corpuscles and played a vital part in the acceptance of the wave theory of light. However subsequent research, involving the discovery of the electron and electromagnetic radiation , would lead to scientists considering yet again that light behaved as a particle too, thus giving rise to wave-particle duality theory.

Electromagnetism and Special Relativity:

Prior to the 19th and 20th centuries, the speed of light had already been determined. The first recorded measurements were performed by Danish astronomer Ole Rømer, who demonstrated in 1676 using light measurements from Jupiter's moon Io to show that light travels at a finite speed (rather than instantaneously).

By the late 19th century , James Clerk Maxwell proposed that light was an electromagnetic wave, and devised several equations (known as Maxwell's equations) to describe how electric and magnetic fields are generated and altered by each other and by charges and currents. By conducting measurements of different types of radiation (magnetic fields, ultraviolet and infrared radiation), he was able to calculate the speed of light in a vacuum (represented as c).

In 1905, Albert Einstein published "On the Electrodynamics of Moving Bodies", in which he advanced one of his most famous theories and overturned centuries of accepted notions and orthodoxies. In his paper, he postulated that the speed of light was the same in all inertial reference frames, regardless of the motion of the light source or the position of the observer.

Exploring the consequences of this theory is what led him to propose his theory of Special Relativity, which reconciled Maxwell's equations for electricity and magnetism with the laws of mechanics, simplified the mathematical calculations, and accorded with the directly observed speed of light and accounted for the observed aberrations. It also demonstrated that the speed of light had relevance outside the context of light and electromagnetism.

For one, it introduced the idea that major changes occur when things move close the speed of light, including the time-space frame of a moving body appearing to slow down and contract in the direction of motion when measured in the frame of the observer. After centuries of increasingly precise measurements, the speed of light was determined to be 299,792,458 m/s in 1975.

Einstein and the Photon:

In 1905, Einstein also helped to resolve a great deal of confusion surrounding the behavior of electromagnetic radiation when he proposed that electrons are emitted from atoms when they absorb energy from light. Known as the photoelectric effect, Einstein based his idea on Planck's earlier work with "black bodies" – materials that absorb electromagnetic energy instead of reflecting it (i.e. white bodies).

At the time, Einstein's photoelectric effect was attempt to explain the "black body problem", in which a black body emits electromagnetic radiation due to the object's heat. This was a persistent problem in the world of physics, arising from the discovery of the electron, which had only happened eight years previous (thanks to British physicists led by J.J. Thompson and experiments using cathode ray tubes).

At the time, scientists still believed that electromagnetic energy behaved as a wave, and were therefore hoping to be able to explain it in terms of classical physics. Einstein's explanation represented a break with this, asserting that electromagnetic radiation behaved in ways that were consistent with a particle – a quantized form of light which he named "photons". For this discovery, Einstein was awarded the Nobel Prize in 1921.

Wave-Particle Duality:

Subsequent theories on the behavior of light would further refine this idea, which included French physicist Louis-Victor de Broglie calculating the wavelength at which light functioned. This was followed by Heisenberg's "uncertainty principle" (which stated that measuring the position of a photon accurately would disturb measurements of it momentum and vice versa), and Schrödinger's paradox that claimed that all particles have a " wave function ".

In accordance with quantum mechanical explanation, Schrodinger proposed that all the information about a particle (in this case, a photon) is encoded in its wave function, a complex-valued function roughly analogous to the amplitude of a wave at each point in space. At some location, the measurement of the wave function will randomly "collapse", or rather "decohere", to a sharply peaked function. This was illustrated in Schrödinger famous paradox involving a closed box, a cat, and a vial of poison (known as the "Schrödinger's Cat" paradox).

According to his theory, wave function also evolves according to a differential equation (aka. the Schrödinger equation). For particles with mass, this equation has solutions; but for particles with no mass, no solution existed. Further experiments involving the Double-Slit Experiment confirmed the dual nature of photons. where measuring devices were incorporated to observe the photons as they passed through the slits.

When this was done, the photons appeared in the form of particles and their impacts on the screen corresponded to the slits – tiny particle-sized spots distributed in straight vertical lines. By placing an observation device in place, the wave function of the photons collapsed and the light behaved as classical particles once more. As predicted by Schrödinger, this could only be resolved by claiming that light has a wave function, and that observing it causes the range of behavioral possibilities to collapse to the point where its behavior becomes predictable.

The development of Quantum Field Theory (QFT) was devised in the following decades to resolve much of the ambiguity around wave-particle duality. And in time, this theory was shown to apply to other particles and fundamental forces of interaction (such as weak and strong nuclear forces). Today, photons are part of the Standard Model of particle physics, where they are classified as boson – a class of subatomic particles that are force carriers and have no mass.

So how does light travel? Basically, traveling at incredible speeds (299 792 458 m/s) and at different wavelengths, depending on its energy. It also behaves as both a wave and a particle, able to propagate through mediums (like air and water) as well as space. It has no mass, but can still be absorbed, reflected, or refracted if it comes in contact with a medium. And in the end, the only thing that can truly slow down or arrest the speed of light is gravity (i.e. a black hole).

What we have learned about light and electromagnetism has been intrinsic to the revolution which took place in physics in the early 20th century, a revolution that we have been grappling with ever since. Thanks to the efforts of scientists like Maxwell, Planck, Einstein, Heisenberg and Schrodinger, we have learned much, but still have much to learn.

For instance, its interaction with gravity (along with weak and strong nuclear forces) remains a mystery. Unlocking this, and thus discovering a Theory of Everything (ToE) is something astronomers and physicists look forward to. Someday, we just might have it all figured out!

Source: Universe Today

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• Light Travels in Straight Line

An Introduction

Light is one form of energy that plays a vital role in our life. We cannot imagine a world full of darkness. Light makes our vision possible and enhances the beauty of everything around us. Light is playing an important role in both art and science. Light is one of the important tools in science that helps scientists to observe things around the world.

Some theories of science are saying it is particles and some of them are saying light is a wave . If the light is a wave, how does light travel and what is the medium of propagation? Light travels in a straight line. The straight-line path of light is very much evident when light travels through a dusty atmosphere. In this article, we will be discussing the straight line motion of light.

How does Light Travel?

Light can travel through both in a medium and in a vacuum . But in a vacuum, there will not be any particles light can not reflect by hitting it. Hence, in a vacuum, light is invisible. In air, light can be reflected by hitting dust or some other particles, hence light is visible in the air.

Light can be considered as waves. Light waves travel in different wavelengths and depending on the wavelength, different light has different colours. For example, the high wavelength light in visible light has a red colour and the shortest wavelength of light has a violet colour. Being a wave light can show properties of waves such as interference and diffraction .

The answer to the question of how light normally travels is that light travels in a straight line. But the actual answer is light seems to travel in a straight line because of the smaller diffraction effect of light. Diffraction is the bending of waves around an object such that it spreads out and illuminates an area where a shadow is expected.

For light, the wavelength is in the order of nanometers. This wavelength is too small and obstacles of this size cannot be determined by our naked eyes. Hence, we feel that light travels along a straight line. The straight-line motion of light is also called rectilinear propagation of light .

Experiment for the Straight Line Motion of Light

Since the diffraction effect of light is too small, normally light travels along a straight line. By using a simple experimental setup, we can prove that light travels along a straight line.

Place three cardboard sheets back to back in front of a candle on the tabletop. Make sure that the cardboard sheets and the candles are placed in a straight line. Light the candle and make a pinhole on each cardboard sheet. The holes should be made at equal height such that the flame of the candle is visible through them. Now look through the holes and observe light travels in which line. The light flame will be visible along the straight line of holes. Now move one of the cardboard sheets to either side and observe the flame. Can you see the flame? On moving the cardboard sheet, the flame will not be visible. Now, again place the cardboard sheet back in its position. The flame is visible now.

From this experiment, we can conclude that light travels along a straight line and this experiment diagram is given below.

Examples of Straight Line Motion of Light

Light travels in straight line examples are as follows:.

Light comes out from a torch or train or lamp follows a straight line path.

A straight line path of light is visible when Sunlight comes out through the small holes in a dusty atmosphere.

When we place any opaque object in front of the object, we observe that the object will be invisible. It is because light cannot bend through the corners of the opaque object.

Interesting Facts

Sunlight can reach a depth of 80m in the ocean.

Paul Dirac proposed a theory in explaining the dual nature of light in 1927.

Particles of light are called photons.

The scientist Euclid Catoptrics in 280 BC found light travels in straight-line inhomogeneous media.

Key Features

Light travels along a straight line.

The straight-line motion of light is due to its small diffraction effects.

Light comes out from the train, torch, and lamp are examples of straight line motion of light.

FAQs on Light Travels in Straight Line

1. What is rectilinear propagation of light?

Light travels along a straight line. The straight-line motion of light is called rectilinear propagation of light.

2. Explain why light travels in a straight line?

Light is a wave exhibiting the property of diffraction. The phenomenon of diffraction is observed only if the wavelength of the wave matches the size of the particle it collides with. Light has wavelengths in the order of nanometers. Usually, an object of nanometer size can not be seen by the naked eye. Hence, the diffraction effect of light is too small to be considered. So, light appears to travel along a straight line.

3. Why is light invisible in a vacuum?

Light can travel through a vacuum. Since in a vacuum there are no particles, light can not reflect. Hence, light is invisible in a vacuum.

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How to Prove That Light Travels in a Straight Path

Last Updated: April 24, 2024 Fact Checked

This article was co-authored by Chris Hasegawa, PhD . Dr. Chris Hasegawa was a Science Professor and the Dean at California State University Monterey Bay. Dr. Hasegawa specializes in teaching complex scientific concepts to students. He holds a BS in Biochemistry, a Master’s in Education, and his teaching credential from The University of California, Davis. He earned his PhD in Curriculum and Instruction from The University of Oregon. Before becoming a professor, Dr. Hasegawa conducted biochemical research in Neuropharmacology at the National Institute of Health. He also taught physical and life sciences and served as a teacher and administrator at public schools in California, Oregon, and Arizona. This article has been fact-checked, ensuring the accuracy of any cited facts and confirming the authority of its sources. This article has been viewed 213,161 times.

Light is an essential part of your day. It allows you to see objects, shapes, and colors. In fact, the pupils in your eyes filter in light to help you see everything around you. As part of a school assignment, you may be asked to prove that light travels in a straight line. You can do this using basic household items in three easy experiments.

Making a Light Pinhole

• Three index cards.
• A piece of modeling clay or sticky tack. You can also use double sided tape.

• A hole puncher.

• Take the hole puncher and punch a hole at the center of the card where the two lines intersect. Do this for the other two cards.

• Form a stand for the cards using the clay so the cards are straight and upright. Use the ruler to ensure the cards are two to five inches from each other.
• You can also use double sided tape to attach the cards to a surface in a vertical position. Do not cover or obstruct the hole in the center of the cards with modeling clay or tape.

• Note that the light can be seen through all the holes. You should be able to see the light go through all the holes and land on a wall or surface beyond the last index card.

Using a Mirror and a Flashlight

• Two to three sheets of black paper.
• Small objects like buttons, bottle caps, or dimes.

• The other person will use the small mirror to reflect the flashlight so it hits the objects. Move close to the light, at an angle, to catch the light so it hits the objects.
• You may need to position more than one mirror to create a light path that shines on the objects. Play around with reflecting the light on the mirrors until the light hits the objects. You can also move the objects around the room to create a more complicated light path, using the flashlight as the light source.
• This experiment shows that light travels in a straight line in the air. But it also bounces off of a reflective surface, like a mirror. The angle of the light as it bounces off the mirror will be the same as the angle of the light as it hits the mirror. The mirror reflects the light and changes its path from a straight line to an angled straight line.

Using Water and Oil

• A large glass jar.
• Access to water.
• One cup of oil.

• Make sure the jar is large enough to fit the ruler.

• Note that the numbers appear stretched or magnified as the light rays bend in the oil and the water. Move the ruler from side to side to note the different appearances of the ruler numbers in the oil and in the water.
• This will show that light travels at different speeds in different mediums, such as air, oil, and water. It will travel in a straight line in the air, but it will bend when it changes speed due to contact with a certain medium, like oil or water.

Expert Q&A

Things You'll Need

• A piece of modeling clay or sticky tack. You can also use tape.
• A flashlight or a laser pointer.
• A flashlight.
• A small mirror.

You Might Also Like

• ↑ http://www.ducksters.com/science/experiment_light_travel.php
• ↑ Chris Hasegawa, PhD. Retired Science Professor & Dean. Expert Interview. 29 July 2021.
• ↑ https://www.science-sparks.com/science-fair-projects-light-maze/
• ↑ https://www.scientificamerican.com/article/now-you-see-it-testing-out-light-refraction/

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Light Travels Along a Straight Line

Introduction.

One type of energy that is essential to our existence is light. We are unable to envision a world without light. Light improves the beauty of everything around us and allows us to see. In both science and art, light is a crucial element. One of the crucial scientific instruments that enable scientists to examine things all across the world is light.

Some scientific theories claim that it is made up of particles, while others assert that it is made up of waves. What is the medium of propagation if the light is a wave? How does light move? We shall find answers to some of these questions in this article.

How does Light Travel?

Light can pass through a medium and in a vacuum. However, there won’t be any particles in a vacuum that light can’t reflect off of. Therefore, light is invisible in a vacuum. Light can reflect in the air when it strikes dust or other particles, making light visible in the atmosphere. Light may be thought of as having waves. Different light waves have varied wavelengths, and different light has different colours based on the wavelength. For instance, the shortest wavelength of light has a violet colour, whereas the highest wavelength of visible light has a red colour. Light, being a wave, may exhibit wave characteristics like diffraction and interference.

The answer to the question of how light typically moves is that it moves straightforwardly. However, the truth is that light’s smaller diffraction effect is the reason it appears to move in a straight line. The spreading out and the illumination of an area where a shadow is anticipated is known as diffraction, which is the bending of waves around an object. The wavelength of light is on the order of nanometers. We cannot see impediments of this size with our unaided eyes because the wavelength is too narrow. As a result, we see that light moves in a straight path. Rectilinear propagation of light is another name for the way that light moves in a straight line.

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Experiment with the Straight Line Motion of Light

Normal light travels in a straight line because there isn’t enough diffraction to cause any noticeable effects. We demonstrate that light moves in a straight line using a basic experimental setup. In front of a candle on the tabletop, arrange three cardboard sheets back to back. Ensure that the candles and cardboard sheets are arranged in a straight line. On each cardboard sheet, poke a pinhole after lighting the candle. To allow for the visibility of the candle’s flame, the holes must be made at equal heights. Now, observe which line light travels in by looking through the holes. Along the slender line of holes, the thin flame will be visible. Now move one of the cardboard sheets to either side and observe the flame. Can you see the flame? The flame won’t be seen when you move the cardboard sheet. Reposition the cardboard sheet in its original location. The flame may now be seen. The experiment diagram is shown below. From this experiment, we may infer that light moves in a straight line.

Examples of Straight-Line Motion of Light

• When a lamp, torch, or another source of light emits light, it travels in a straight line.
• When sunlight enters a dusty environment through tiny holes, a straight-line trail of light is apparent.
• The object will become invisible when an opaque object is placed in front of it. The reason for this is that an opaque object prevents light from bending through its corners.

The light rays move in a straight line. The minimized diffraction effects of light facilitate the propagation of the light in a straight path. Examples, where the light rays travel in a straight line, are the light ray that comes from a train, a torch, and/or a lamp.

Frequently Asked Questions

1. what is rectilinear propagation of light.

Ans: The motion of the light rays in a straight line is termed the rectilinear propagation of the light.

2. Explain why Light Travels in a Straight Line?

Ans: Diffraction is a wave characteristic of light. Only when the wavelength of the light wave is of the order of the dimension of the size of the particle it collides with, and the phenomena of diffraction take place. The wavelengths of light are on the order of nanometers. Typically, a nanometer-sized item is invisible to the human eye. Light’s diffraction impact is therefore too modest to be taken into account and travels in a straight-line path.

3. Why is Light Invisible in a Vacuum?

Ans: Light can travel through the vacuum, however, since there are no particles available in the vacuum, light cannot reflect in a vacuum and therefore light is invisible in a vacuum.

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RECENT POSTS

What is the speed of light?

The speed of light is the speed limit of the universe. Or is it?

What is a light-year?

• Speed of light FAQs
• Special relativity
• Faster than light
• Slowing down light
• Faster-than-light travel

Bibliography

The speed of light traveling through a vacuum is exactly 299,792,458 meters (983,571,056 feet) per second. That's about 186,282 miles per second — a universal constant known in equations as "c," or light speed. 

According to physicist Albert Einstein 's theory of special relativity , on which much of modern physics is based, nothing in the universe can travel faster than light. The theory states that as matter approaches the speed of light, the matter's mass becomes infinite. That means the speed of light functions as a speed limit on the whole universe . The speed of light is so immutable that, according to the U.S. National Institute of Standards and Technology , it is used to define international standard measurements like the meter (and by extension, the mile, the foot and the inch). Through some crafty equations, it also helps define the kilogram and the temperature unit Kelvin .

But despite the speed of light's reputation as a universal constant, scientists and science fiction writers alike spend time contemplating faster-than-light travel. So far no one's been able to demonstrate a real warp drive, but that hasn't slowed our collective hurtle toward new stories, new inventions and new realms of physics.

Related: Special relativity holds up to a high-energy test

A l ight-year is the distance that light can travel in one year — about 6 trillion miles (10 trillion kilometers). It's one way that astronomers and physicists measure immense distances across our universe.

Light travels from the moon to our eyes in about 1 second, which means the moon is about 1 light-second away. Sunlight takes about 8 minutes to reach our eyes, so the sun is about 8 light minutes away. Light from Alpha Centauri , which is the nearest star system to our own, requires roughly 4.3 years to get here, so Alpha Centauri is 4.3 light-years away.

"To obtain an idea of the size of a light-year, take the circumference of the Earth (24,900 miles), lay it out in a straight line, multiply the length of the line by 7.5 (the corresponding distance is one light-second), then place 31.6 million similar lines end to end," NASA's Glenn Research Center says on its website . "The resulting distance is almost 6 trillion (6,000,000,000,000) miles!"

Stars and other objects beyond our solar system lie anywhere from a few light-years to a few billion light-years away. And everything astronomers "see" in the distant universe is literally history. When astronomers study objects that are far away, they are seeing light that shows the objects as they existed at the time that light left them. 

This principle allows astronomers to see the universe as it looked after the Big Bang , which took place about 13.8 billion years ago. Objects that are 10 billion light-years away from us appear to astronomers as they looked 10 billion years ago — relatively soon after the beginning of the universe — rather than how they appear today.

Related: Why the universe is all history

Speed of light FAQs answered by an expert

We asked Rob Zellem, exoplanet-hunter and staff scientist at NASA's Jet Propulsion Lab, a few frequently asked questions about the speed of light. 

Dr. Rob Zellem is a staff scientist at NASA's Jet Propulsion Laboratory, a federally funded research and development center operated by the California Institute of Technology. Rob is the project lead for Exoplanet Watch, a citizen science project to observe exoplanets, planets outside of our own solar system, with small telescopes. He is also the Science Calibration lead for the Nancy Grace Roman Space Telescope's Coronagraph Instrument, which will directly image exoplanets. 

What is faster than the speed of light?

Nothing! Light is a "universal speed limit" and, according to Einstein's theory of relativity, is the fastest speed in the universe: 300,000 kilometers per second (186,000 miles per second). 

Is the speed of light constant?

The speed of light is a universal constant in a vacuum, like the vacuum of space. However, light *can* slow down slightly when it passes through an absorbing medium, like water (225,000 kilometers per second = 140,000 miles per second) or glass (200,000 kilometers per second = 124,000 miles per second). 

Who discovered the speed of light?

One of the first measurements of the speed of light was by Rømer in 1676 by observing the moons of Jupiter . The speed of light was first measured to high precision in 1879 by the Michelson-Morley Experiment. 

How do we know the speed of light?

Rømer was able to measure the speed of light by observing eclipses of Jupiter's moon Io. When Jupiter was closer to Earth, Rømer noted that eclipses of Io occurred slightly earlier than when Jupiter was farther away. Rømer attributed this effect due the time it takes for light to travel over the longer distance when Jupiter was farther from the Earth. 

How did we learn the speed of light?

As early as the 5th century BC, Greek philosophers like Empedocles and Aristotle disagreed on the nature of light speed. Empedocles proposed that light, whatever it was made of, must travel and therefore, must have a rate of travel. Aristotle wrote a rebuttal of Empedocles' view in his own treatise, On Sense and the Sensible , arguing that light, unlike sound and smell, must be instantaneous. Aristotle was wrong, of course, but it would take hundreds of years for anyone to prove it. 

In the mid 1600s, the Italian astronomer Galileo Galilei stood two people on hills less than a mile apart. Each person held a shielded lantern. One uncovered his lantern; when the other person saw the flash, he uncovered his too. But Galileo's experimental distance wasn't far enough for his participants to record the speed of light. He could only conclude that light traveled at least 10 times faster than sound.

In the 1670s, Danish astronomer Ole Rømer tried to create a reliable timetable for sailors at sea, and according to NASA , accidentally came up with a new best estimate for the speed of light. To create an astronomical clock, he recorded the precise timing of the eclipses of Jupiter's moon , Io, from Earth . Over time, Rømer observed that Io's eclipses often differed from his calculations. He noticed that the eclipses appeared to lag the most when Jupiter and Earth were moving away from one another, showed up ahead of time when the planets were approaching and occurred on schedule when the planets were at their closest or farthest points. This observation demonstrated what we today know as the Doppler effect, the change in frequency of light or sound emitted by a moving object that in the astronomical world manifests as the so-called redshift , the shift towards "redder", longer wavelengths in objects speeding away from us. In a leap of intuition, Rømer determined that light was taking measurable time to travel from Io to Earth. 

Rømer used his observations to estimate the speed of light. Since the size of the solar system and Earth's orbit wasn't yet accurately known, argued a 1998 paper in the American Journal of Physics , he was a bit off. But at last, scientists had a number to work with. Rømer's calculation put the speed of light at about 124,000 miles per second (200,000 km/s).

In 1728, English physicist James Bradley based a new set of calculations on the change in the apparent position of stars caused by Earth's travels around the sun. He estimated the speed of light at 185,000 miles per second (301,000 km/s) — accurate to within about 1% of the real value, according to the American Physical Society .

Two new attempts in the mid-1800s brought the problem back to Earth. French physicist Hippolyte Fizeau set a beam of light on a rapidly rotating toothed wheel, with a mirror set up 5 miles (8 km) away to reflect it back to its source. Varying the speed of the wheel allowed Fizeau to calculate how long it took for the light to travel out of the hole, to the adjacent mirror, and back through the gap. Another French physicist, Leon Foucault, used a rotating mirror rather than a wheel to perform essentially the same experiment. The two independent methods each came within about 1,000 miles per second (1,609 km/s) of the speed of light.

Another scientist who tackled the speed of light mystery was Poland-born Albert A. Michelson, who grew up in California during the state's gold rush period, and honed his interest in physics while attending the U.S. Naval Academy, according to the University of Virginia . In 1879, he attempted to replicate Foucault's method of determining the speed of light, but Michelson increased the distance between mirrors and used extremely high-quality mirrors and lenses. Michelson's result of 186,355 miles per second (299,910 km/s) was accepted as the most accurate measurement of the speed of light for 40 years, until Michelson re-measured it himself. In his second round of experiments, Michelson flashed lights between two mountain tops with carefully measured distances to get a more precise estimate. And in his third attempt just before his death in 1931, according to the Smithsonian's Air and Space magazine, he built a mile-long depressurized tube of corrugated steel pipe. The pipe simulated a near-vacuum that would remove any effect of air on light speed for an even finer measurement, which in the end was just slightly lower than the accepted value of the speed of light today. 

Michelson also studied the nature of light itself, wrote astrophysicist Ethan Siegal in the Forbes science blog, Starts With a Bang . The best minds in physics at the time of Michelson's experiments were divided: Was light a wave or a particle? 

Michelson, along with his colleague Edward Morley, worked under the assumption that light moved as a wave, just like sound. And just as sound needs particles to move, Michelson and Morley and other physicists of the time reasoned, light must have some kind of medium to move through. This invisible, undetectable stuff was called the "luminiferous aether" (also known as "ether"). 

Though Michelson and Morley built a sophisticated interferometer (a very basic version of the instrument used today in LIGO facilities), Michelson could not find evidence of any kind of luminiferous aether whatsoever. Light, he determined, can and does travel through a vacuum.

"The experiment — and Michelson's body of work — was so revolutionary that he became the only person in history to have won a Nobel Prize for a very precise non-discovery of anything," Siegal wrote. "The experiment itself may have been a complete failure, but what we learned from it was a greater boon to humanity and our understanding of the universe than any success would have been!"

Special relativity and the speed of light

Einstein's theory of special relativity unified energy, matter and the speed of light in a famous equation: E = mc^2. The equation describes the relationship between mass and energy — small amounts of mass (m) contain, or are made up of, an inherently enormous amount of energy (E). (That's what makes nuclear bombs so powerful: They're converting mass into blasts of energy.) Because energy is equal to mass times the speed of light squared, the speed of light serves as a conversion factor, explaining exactly how much energy must be within matter. And because the speed of light is such a huge number, even small amounts of mass must equate to vast quantities of energy.

In order to accurately describe the universe, Einstein's elegant equation requires the speed of light to be an immutable constant. Einstein asserted that light moved through a vacuum, not any kind of luminiferous aether, and in such a way that it moved at the same speed no matter the speed of the observer. 

Think of it like this: Observers sitting on a train could look at a train moving along a parallel track and think of its relative movement to themselves as zero. But observers moving nearly the speed of light would still perceive light as moving away from them at more than 670 million mph. (That's because moving really, really fast is one of the only confirmed methods of time travel — time actually slows down for those observers, who will age slower and perceive fewer moments than an observer moving slowly.)

In other words, Einstein proposed that the speed of light doesn't vary with the time or place that you measure it, or how fast you yourself are moving. 

Therefore, objects with mass cannot ever reach the speed of light. If an object ever did reach the speed of light, its mass would become infinite. And as a result, the energy required to move the object would also become infinite: an impossibility.

That means if we base our understanding of physics on special relativity (which most modern physicists do), the speed of light is the immutable speed limit of our universe — the fastest that anything can travel. 

What goes faster than the speed of light?

Although the speed of light is often referred to as the universe's speed limit, the universe actually expands even faster. The universe expands at a little more than 42 miles (68 kilometers) per second for each megaparsec of distance from the observer, wrote astrophysicist Paul Sutter in a previous article for Space.com . (A megaparsec is 3.26 million light-years — a really long way.) 

In other words, a galaxy 1 megaparsec away appears to be traveling away from the Milky Way at a speed of 42 miles per second (68 km/s), while a galaxy two megaparsecs away recedes at nearly 86 miles per second (136 km/s), and so on. 

"At some point, at some obscene distance, the speed tips over the scales and exceeds the speed of light, all from the natural, regular expansion of space," Sutter explained. "It seems like it should be illegal, doesn't it?"

Special relativity provides an absolute speed limit within the universe, according to Sutter, but Einstein's 1915 theory regarding general relativity allows different behavior when the physics you're examining are no longer "local."

"A galaxy on the far side of the universe? That's the domain of general relativity, and general relativity says: Who cares! That galaxy can have any speed it wants, as long as it stays way far away, and not up next to your face," Sutter wrote. "Special relativity doesn't care about the speed — superluminal or otherwise — of a distant galaxy. And neither should you."

Does light ever slow down?

Light in a vacuum is generally held to travel at an absolute speed, but light traveling through any material can be slowed down. The amount that a material slows down light is called its refractive index. Light bends when coming into contact with particles, which results in a decrease in speed.

For example, light traveling through Earth's atmosphere moves almost as fast as light in a vacuum, slowing down by just three ten-thousandths of the speed of light. But light passing through a diamond slows to less than half its typical speed, PBS NOVA reported. Even so, it travels through the gem at over 277 million mph (almost 124,000 km/s) — enough to make a difference, but still incredibly fast.

Light can be trapped — and even stopped — inside ultra-cold clouds of atoms, according to a 2001 study published in the journal Nature . More recently, a 2018 study published in the journal Physical Review Letters proposed a new way to stop light in its tracks at "exceptional points," or places where two separate light emissions intersect and merge into one.

Researchers have also tried to slow down light even when it's traveling through a vacuum. A team of Scottish scientists successfully slowed down a single photon, or particle of light, even as it moved through a vacuum, as described in their 2015 study published in the journal Science . In their measurements, the difference between the slowed photon and a "regular" photon was just a few millionths of a meter, but it demonstrated that light in a vacuum can be slower than the official speed of light. 

Can we travel faster than light?

— Spaceship could fly faster than light

— Here's what the speed of light looks like in slow motion

— Why is the speed of light the way it is?

Science fiction loves the idea of "warp speed." Faster-than-light travel makes countless sci-fi franchises possible, condensing the vast expanses of space and letting characters pop back and forth between star systems with ease. 

But while faster-than-light travel isn't guaranteed impossible, we'd need to harness some pretty exotic physics to make it work. Luckily for sci-fi enthusiasts and theoretical physicists alike, there are lots of avenues to explore.

All we have to do is figure out how to not move ourselves — since special relativity would ensure we'd be long destroyed before we reached high enough speed — but instead, move the space around us. Easy, right? 

One proposed idea involves a spaceship that could fold a space-time bubble around itself. Sounds great, both in theory and in fiction.

"If Captain Kirk were constrained to move at the speed of our fastest rockets, it would take him a hundred thousand years just to get to the next star system," said Seth Shostak, an astronomer at the Search for Extraterrestrial Intelligence (SETI) Institute in Mountain View, California, in a 2010 interview with Space.com's sister site LiveScience . "So science fiction has long postulated a way to beat the speed of light barrier so the story can move a little more quickly."

Without faster-than-light travel, any "Star Trek" (or "Star War," for that matter) would be impossible. If humanity is ever to reach the farthest — and constantly expanding — corners of our universe, it will be up to future physicists to boldly go where no one has gone before.

Additional resources

For more on the speed of light, check out this fun tool from Academo that lets you visualize how fast light can travel from any place on Earth to any other. If you’re more interested in other important numbers, get familiar with the universal constants that define standard systems of measurement around the world with the National Institute of Standards and Technology . And if you’d like more on the history of the speed of light, check out the book " Lightspeed: The Ghostly Aether and the Race to Measure the Speed of Light " (Oxford, 2019) by John C. H. Spence.

Aristotle. “On Sense and the Sensible.” The Internet Classics Archive, 350AD. http://classics.mit.edu/Aristotle/sense.2.2.html .

D’Alto, Nick. “The Pipeline That Measured the Speed of Light.” Smithsonian Magazine, January 2017. https://www.smithsonianmag.com/air-space-magazine/18_fm2017-oo-180961669/ .

Fowler, Michael. “Speed of Light.” Modern Physics. University of Virginia. Accessed January 13, 2022. https://galileo.phys.virginia.edu/classes/252/spedlite.html#Albert%20Abraham%20Michelson .

Giovannini, Daniel, Jacquiline Romero, Václav Potoček, Gergely Ferenczi, Fiona Speirits, Stephen M. Barnett, Daniele Faccio, and Miles J. Padgett. “Spatially Structured Photons That Travel in Free Space Slower than the Speed of Light.” Science, February 20, 2015. https://www.science.org/doi/abs/10.1126/science.aaa3035 .

Goldzak, Tamar, Alexei A. Mailybaev, and Nimrod Moiseyev. “Light Stops at Exceptional Points.” Physical Review Letters 120, no. 1 (January 3, 2018): 013901. https://doi.org/10.1103/PhysRevLett.120.013901 . 

Hazen, Robert. “What Makes Diamond Sparkle?” PBS NOVA, January 31, 2000. https://www.pbs.org/wgbh/nova/article/diamond-science/ . 

“How Long Is a Light-Year?” Glenn Learning Technologies Project, May 13, 2021. https://www.grc.nasa.gov/www/k-12/Numbers/Math/Mathematical_Thinking/how_long_is_a_light_year.htm . 

American Physical Society News. “July 1849: Fizeau Publishes Results of Speed of Light Experiment,” July 2010. http://www.aps.org/publications/apsnews/201007/physicshistory.cfm . 

Liu, Chien, Zachary Dutton, Cyrus H. Behroozi, and Lene Vestergaard Hau. “Observation of Coherent Optical Information Storage in an Atomic Medium Using Halted Light Pulses.” Nature 409, no. 6819 (January 2001): 490–93. https://doi.org/10.1038/35054017 . 

NIST. “Meet the Constants.” October 12, 2018. https://www.nist.gov/si-redefinition/meet-constants . 

Ouellette, Jennifer. “A Brief History of the Speed of Light.” PBS NOVA, February 27, 2015. https://www.pbs.org/wgbh/nova/article/brief-history-speed-light/ . 

Shea, James H. “Ole Ro/Mer, the Speed of Light, the Apparent Period of Io, the Doppler Effect, and the Dynamics of Earth and Jupiter.” American Journal of Physics 66, no. 7 (July 1, 1998): 561–69. https://doi.org/10.1119/1.19020 . 

Siegel, Ethan. “The Failed Experiment That Changed The World.” Forbes, April 21, 2017. https://www.forbes.com/sites/startswithabang/2017/04/21/the-failed-experiment-that-changed-the-world/ . 

Stern, David. “Rømer and the Speed of Light,” October 17, 2016. https://pwg.gsfc.nasa.gov/stargaze/Sun4Adop1.htm . 

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Vicky Stein is a science writer based in California. She has a bachelor's degree in ecology and evolutionary biology from Dartmouth College and a graduate certificate in science writing from the University of California, Santa Cruz (2018). Afterwards, she worked as a news assistant for PBS NewsHour, and now works as a freelancer covering anything from asteroids to zebras. Follow her most recent work (and most recent pictures of nudibranchs) on Twitter. 

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How does light travel in a straight line?

Explanation: light moves in a straight line, as any physics student is aware. but now, scientists have demonstrated that light can curve without any outside help. although the researchers claim it might be used in real-world applications like remotely controlling items with light, the result is essentially an optical illusion. the main reason why light moves in straight lines are because it is a wave and prefers to travel the smallest distance between the two points. light, however, can diverge from a straight trajectory when it strikes certain obstructions. diffraction is a popular name for this phenomenon. light is frequently considered to move in a straight line since diffraction has such a negligible effect in the real life..

Light travels in a straight line.

Why does light appear to travel in a straight line?

Under what special conditions, light does not travel in a straight line?

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If you've ever found yourself waking up in a hotel room with a puffy face post-travel day, or even if you just love the convenience of skincare devices that don't take up the entirety of your vanity cabinet, I've got some news you don't want to miss. Solawave, the brand loved by celebs like Pedro Pascal and Meghann Fahy for the red light wand that transformed one PureWow editor's skin , just released a mini, travel-sized device. And having personally tested it out, I can confirm you're gonna want to add it to your beauty routine ASAP.

The palm-sized Radiant Renewal 2-in-1 Skincare Mini device ($89) integrates red light therapy, near-infrared light and therapeutic warmth to reduce inflammation, tackle signs of aging like fine lines and wrinkles, increase your skin's radiance and decrease inflammation (but more on all that in a second) on both the face and body. The point is, this small device seriously packs a punch, and it comes in at just under $90, meaning you won't have to break the bank to grab one for yourself. Below, find all the need-to-knows about the Skincare Mini, plus my own thoughts after trying it.

Want to know which buzzy products are *really* worth buying?  Sign up for our shopping newsletter  to uncover our favorite finds.

What Are the Benefits of Red Light Therapy, Near-Infrared Light and Therapeutic Warmth?

• Red Light Therapy: Per Cleveland Clinic , red light therapy, which is a treatment that uses low levels of red light on your skin, has shown promise in ongoing research for treating wrinkles, redness, acne, scars and signs of aging.
• Near-Infrared Light: Rupa Health shares that when infrared light is used on the skin, it kicks cells into high gear and promotes tissue healing, inflammation reduction and pain relief.
• Therapeutic Warmth: Otherwise known as heat therapy, applying warmth to the skin can aid in increasing blood flow and circulation through the skin by relaxing the muscles and dilating blood vessels, per the brand. This also then allows oxygen and nutrients to flow more freely to help with tissue regrowth and lymphatic drainage.

How to Use the Solawave Skincare Mini

Unlike the OG Radiant Renewal wand (was $198, now $132), the Solawave Skincare Mini can be used on both your face  and body since it doesn't include galvanic current(though the brand recommends a patch test on a small area of skin like your wrist, first), and should be used on clean, dry skin. You can use the device with or without the Skin Therapy Activating serum (was $29, now $20), which is sold separately, and you simply hold down the power button to get it started. Then, glide the device in circular motions for three minutes on one focal area before moving onto another. Your total treatment should take no longer than 12 minutes, after which the device will automatically shut off. The brand recommends cleaning after each treatment with an alcohol- or water-based wipe.

It's also important to note that this device should not be used by anyone that is currently pregnant, as it hasn't been tested for pregnancy safety, and shouldn't be used by anyone under the age of 18, subject to seizures or photosensitivity, that has skin cancer or with a known allergy to aluminum, silicone or plastic.

All My Thoughts on the Travel-Sized Device

When I first unpacked the Skincare Mini, I first noticed just how compact it is—truly the perfect travel size. I was also happy to see detailed instructions were included, so I wasn't nervous to jump in and try it out.

Let's get right into a few things I loved about the mini device. First, it heats up quickly to a low warmth, so there's no waiting around for it to be ready to use—you can just turn it on and get going. Plus, it's designed with a PopSocket -like handle, making it really easy to angle on your face without dropping it. And as someone who tends to be super impatient with my skincare (you won't ever find me spending hours applying different products or using time-consuming devices), I also loved that I could just toss on an audiobook and it felt like the 12 minute treatment was over in a flash. As for the results, though it's a little hard to see in the photos above, I definitely noticed my face was less puffy and looked a bit brighter overall immediately after using it.

Now, on to what I didn't love. The biggest thing here is that, though, the Skincare Mini is designed for the light to blink when it's time to move onto a new focal area, I found that when I used it on my face, I wasn't able to see the blinking. So for my first session, I ended up using it way too long on the first side before switching. This is easily remedied by just watching the clock while you're using the device on your face (it's easy to notice on other areas of your body like your arm), but definitely something to note. Also, the light can be pretty bright when it gets closer to your eyes, such as when targeting the cheekbones or lower forehead, so keep that in mind, especially if you have sensitive eyes.

The Bottom Line

Overall, I think this device is totally worth it's less than $90 price tag—especially if you're someone (like me) who tends to end up with a puffy face post-travel or early in the mornings and want to have a quick fix on hand that doesn't take up a ton of your time or space. Considering the celebrity following of the brand, I have a feeling this new release could sell out quickly, so you might want to jump on it sooner than later.

I Tried The Solawave Radiant Renewal Wand and It Transformed My Skin Texture in Just 2 Weeks (Plus, It's on Sale)

Assistant Commerce Editor

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Opinion | Baltimore Red Line alternatives all come up…

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Opinion | Baltimore Red Line alternatives all come up short | READER COMMENTARY

Alternates 1 and 3 put light rail in a tunnel under Pratt Street right next to the harbor. Such a route will likely have challenges and cost overruns during construction. It is the fastest of the alternatives, but since the plan uses light rail cars it will have limited capacity, and since it uses tunnels it will have few stations. The map shows it ending near Fells Point. If the tunnel has expensive leaking problems once constructed or if it has the sort of cost overruns that a tunnel going through a site that was originally swamp, then the city and its taxpayers will really regret this option. It can be done, but it will be gambling with the taxpayer’s money.

Alternates 2A and 4A use a surface route through downtown, possibly with a tunnel on the west side. This is the least expensive solution. But for people familiar with these roads adding Bus Rapid Transit or BRT means either losing a lane of traffic or having the bus not be “rapid” at all — just another bus route in a city full of bus routes that are slower than a kid on a scooter or skateboard can travel during peak travel times. And since it will clog already clogged streets, there will be lots of unhappy people.

Alternates 2B and 4B are similar to 2A and 4A. Using light rail instead of buses could make for slightly higher speed and capacity, but can also lead to clogging some of the city’s most congested roads. I suspect that any of the surface options will be abandoned within 20 years unless someone invents inexpensive flying cars.

The original Red Line plan expanded the subway system under Baltimore Street. It added a branch splitting off to the southeast past downtown and a branch headed west to make an X layout. That would allow the current subway riders to have a choice of destinations and get there fairly quickly. As a true subway rather than a light rail car in a tunnel, it would have much more capacity. And since there already is a maintenance facility on the subway line would not need another one.

Such an option would be far more expensive since subway cars are more expensive than light rail cars and it would disrupt the subway system during construction. But such a system would likely be far more enduring than a plan to mess up traffic or build right next to water.

— William Hettchen, Ellicott City

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Ukraine war latest: Putin and Kim sign new defence deal - as UK says 'bizarre scenes' should be warning

Vladimir Putin and Kim Jong Un have signed a new deal in Pyongyang, making a vow of mutual aid if either of their countries is attacked as both face escalating stand-offs with the West. Got a question on the Ukraine war? Submit it below for our specialists to answer.

Wednesday 19 June 2024 22:55, UK

• Vladimir Putin visited North Korea for the first time in 24 years today. He has now arrived in Vietnam
• Catch up: Everything you need to know about Putin's visit
• Kim and Putin share 'pent up inmost thoughts'
• Russia and North Korea sign new defence deal
• Analysis: China keeping close eye on cosy friendship
• A limousine, a dagger and artworks - the leaders swap gifts
• 'Bizarre scenes' should be warning, UK says
• Listen to the Daily above and tap here to follow wherever you get your podcasts

Ask a question or make a comment

That's the end of our coverage today, but we'll be back with more updates and analysis on the war in Ukraine soon.

If you're just checking in, here is a recap of the key developments today.

• Vladimir Putin made his first trip to North Korea in 24 years, where he and Kim Jong Un signed an agreement pledging mutual aid if either country faces "aggression";
• The Russian leader was then received in Vietnam, where he will be continue a state visit tomorrow;
• Russia has been attempting to replace "significant losses" by recruiting soldiers from African countries, the UK's defence ministry said;
• Germany held up the 14th package of EU sanctions on Russia, sources said.

Vladimir Putin has undertaken two state visits today, in North Korea and Vietnam, as he seeks to shore up support from his allies while the West pursues further sanctions.

In an opinion piece timed for his Vietnam visit, Vladimir Putin has applauded the country for supporting "a pragmatic way to solve the crisis" in Ukraine. 

Vietnam, which officially pursues a neutral foreign policy, has abstained from condemning Russia's invasion.

As well as praising Vietnam for its "balanced" stance on the war, Mr Putin listed progress on payments, energy and trade between the countries in the article published in Vietnam's Communist Party newspaper Nhan Dan.

"Putin's visit to North Korea and Vietnam is to demonstrate that Western attempts to isolate Russia are not working and that Russia has partners in Asia," said Carl Thayer, an expert on Vietnam security at the Australian Defence Force Academy in Canberra.

The Southeast Asian country will be the third nation Mr Putin has visited, after China and North Korea, since he was sworn in for a fifth term in May.

"No country should give Putin a platform to promote his war of aggression and otherwise allow him to normalise his atrocities," a spokesperson for the US embassy in Hanoi said this week.

Russia has historically been Vietnam's major military supplier, so any potential arms deals will be closely watched.

Vietnam has been gearing up for a full state welcome for the Russian leader's first visit since 2017 and his fifth in total. 

Vladimir Putin has arrived in Vietnam, Russian news agency RIA-Novosti reports.

It is a state visit aimed at strengthening ties with a longtime partner of Moscow's.

In Hanoi, the Russian leader is scheduled to meet Vietnam's most powerful politician, Communist Party general secretary Nguyen Phu Trong, then the new president, To Lam, and other politicians. 

The trip has resulted in a sharp rebuke from the US embassy in Hanoi.

It comes after Mr Putin's first trip to North Korea in 24 years, where he and Kim Jong Un signed an agreement pledging mutual aid if either country faces "aggression". 

Germany is holding up the 14th package of EU sanctions on Russia, sources say.

Officials from the bloc's 27 countries have been debating the package for over a month, which would make EU operators responsible for sanctions violations by subsidiaries and partners.

Germany's hesitation was due in part to an internal disagreement between its foreign ministry and the chancellor's office, diplomats and a source familiar with the matter told Reuters.

The roadblock remained despite the scrapping of a clause that Berlin found problematic - one that would have forced subsidiaries to "contractually prohibit re-exportation to Russia and re-exportation for use in Russia".

The EU's sanctions efforts have been undermined by circumvention via third countries. The inclusion of this clause would have further tightened the bloc's measures. 

Ambassadors will continue the debate tomorrow, they said. 

Volodymyr Zelenskyy has announced two more signatures have been added to his Peace Summit communique. 

He said the Organisation of American States, which represents countries in North and South America, and Antigua and Barbuda have signed it.

"We view the entire world as equals, and this is our ideological difference from Russia in international relations," the Ukrainian president said.

"Putin, however, wants only his voice to matter, or those he chooses. This is a typically colonial world view that we, together with all our partners, must break." 

He said the format of the Peace Summit will ensure "fair treatment for every nation", including, of course, Ukraine. 

Nearly 80 countries have already approved the final communique covering steps toward nuclear safety, food security, and the release of prisoners and deportees, including thousands of children abducted by Russia.

One person has been killed and three others have been injured in a Russian shelling attack in Donetsk, Ukraine's police force has said. 

In a post on Telegram, it said there had been 2,291 strikes in the eastern region in the last 24 hours. 

Russia used "aviation, rocket salvo systems and artillery", it added. 

Over the past few months, fighting has been surging across the frontline in the eastern Donetsk region. 

Russian forces have been trying to reach the key hilltop city of Chasiv Yar and other strategic hubs in the region. 

Russian troops also launched an offensive in Ukraine's northeastern Kharkiv region. 

Mr Putin said he wanted to establish a buffer zone there to prevent Ukrainian cross-border attacks.

The offensive drew some Ukrainian fighters away from Donetsk, but many remain. 

Reuters captured these photos of Ukraine troops operating in the area. 

Russia has been attempting to replace "significant losses" by recruiting soldiers from African countries, the UK's defence ministry has said. 

Ukrainian intelligence has been reporting increased Russian efforts to recruit from several African countries in recent days. 

"This recruitment campaign is likely an attempt to replace significant battlefield losses and sustain offensive activity across the front," the MoD said. 

"Russia is likely expanding its recruitment across the global south to avoid additional mobilisations within Russian itself."

It added that previous mobilisations within Russia had resulted in a record labour shortage and the mass departure of skilled workers. 

Vladimir Putin's visit to North Korea has come to an end, with the Russian president leaving Pyongyang after a busy day of diplomacy.

His next stop is Vietnam.

The headline from Mr Putin's visit is no doubt the agreement signed between the two countries that, its leaders say, covers security, trade, investment and cultural and humanitarian ties among other things.

The deal has triggered plenty of reaction from around the world, with Washington and Seoul among those expressing concern about the growing military ties between Russia and North Korea.

Below, we've taken a look at exactly how Mr Putin's visit shaped up.

Opening remarks

The warm words between the Kremlin and Pyongyang started before Mr Putin's plane had even touched down, with the Russian leader penning an open letter in North Korean state media.

It was here that we first got confirmation of an agreement that would be signed by the two countries. Mr Putin called it a partnership that would help in the "fight against the imperialist hegemonistic policies of the US and its satellites against the Russian Federation".

There was also some appreciation shown for North Korea's "firm support" for Russia's ongoing invasion of Ukraine. 

Mr Putin claimed the US were doing everything they could to "prolong and inflame" the conflict, which Russia began in February 2022.

'Rock star' welcome in Pyongyang

When Mr Putin arrived in Pyongyang in the early hours of the morning, he was met personally by Kim Jong Un, a red carpet and a bouquet of roses.

State television then showed his motorcade driving through brightly lit neighbourhoods, with the 105-storey Ryugyong Hotel glowing with LED lights that said "Welcome Putin".

The streets of the capital were lined with hundreds of thousands of people holding flowers and waving Russian and North Korean flags, welcoming Mr Putin back on to North Korean soil for the first time in 24 years.

There was then a lavish ceremony in Pyongyang's main square, crowded with people and colourful decorations, where Mr Kim introduced key members of the North Korean leadership.

As a military band played, the two leaders walked down another red carpet and greeted dignitaries. Artillery guns fired a welcoming salvo.

As our Asia correspondent Nicole Johnston put it, North Korea gave Mr Putin "a rock star's welcome".

The 'comprehensive strategic partnership pact'

Fundamental to this trip for both Russia and North Korea is the document signed by the two nations, which both of its leaders have been at pains to describe in bold terms.

Mr Kim calls it the "strongest ever treaty", while for Mr Putin, it's a "breakthrough document".

The two leaders were first joined by ministers from both countries for initial talks. They then engaged in one-on-one talks that lasted two hours, according to reports.

This new agreement marks a new period of relations between the two countries, one that, according to North Korea's leader, is "incomparable". 

It reportedly covers everything from investment to strengthening cultural ties and cooperating on health, medical education and science. 

But it's the inclusion of a mutual defence clause, in case of "aggression", as Mr Putin put it, that stands out.

Mr Kim told reporters that North Korea will respond "without hesitation" to "incidents or wars" facing either his country or Russia. 

He did not elaborate on what the response will entail, though he did say the agreement is "defensive and peaceful in nature".

Russia also doesn't rule out the "development" of military-technical cooperation between the two nations. 

There's already an assumption that North Korea is sending Russia ammunition to use in Ukraine, while there are fears that Moscow will pass on aid for Pyongyang's nuclear and missile programmes in return. 

The signing of this agreement will do little to subdue those concerns.

Gifts and goodbyes

With business out the way, Mr Putin and Mr Kim spent the next few hours having a bit of fun.

There was the exchanging of gifts, with the Russian leader receiving works of art depicting himself.

For Mr Kim there was a tea set, an admiral's dirk (a type of dagger) and a Russian-built limousine, which both leaders took turns at driving.

Mr Putin then laid a wreath at the Liberation monument in Pyongyang, which was erected in 1946 in memory of Red Army soldiers killed liberating the Korean Peninsula from Japanese occupation.

Keeping with that theme, both leaders took their seats at a gala concert, where songs performed all evoked war and defence of the nation.

Russian state media reported the concert included excerpts from wartime songs, including Where the Motherland Begins, We Need One Victory and Katyusha, as well as song by groups who regularly support Mr Putin at political rallies.

Putin's opinion piece

As Mr Putin heads for Vietnam, a newspaper in the country has published an opinion piece he has written. 

In the piece, he praised Vietnam for its "balanced" stance on the Ukraine war and applauded the communist-ruled country for supporting "a pragmatic way to solve" the conflict. 

He also said Russia and Vietnam shared "similar assessments of the situation in the Asia-Pacific region".

He is due to arrive in Hanoi overnight and plans to meet Vietnamese leaders tomorrow, on the heels of his trip to North Korea.

It will be his first trip to the country since 2017.

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