do all electromagnetic waves travel at same speed

Do all waves travel at the same speed?

Normal modes on a string, the speed of waves on a string.

Well, strictly speaking, the speed does vary SLIGHTLY -- but by very very small amounts. The graph below shows data based on the measurements listed in L. B. Evans and H. E. Bass, "Tables of absorption and velocity of sound in still air at 68F (20 C)", Technical Report No. AD0738576, Wyle Labs, Huntsville, AL, (1972-01-01), as found at https://apps.dtic.mil/sti/citations/AD0738576 . Click to see the ugly truth Q: If the audience at a music concert sits d = 40 m away from the stage, how large a time delay occurs between notes at f 1 = 100 Hz and f 2 = 1000 Hz ? Would the audience notice?

Radio waves travelling through a plasma

Using dispersion to guess a distance, for more information.

15.2 The Behavior of Electromagnetic Radiation

Section learning objectives.

By the end of this section, you will be able to do the following:

• Describe the behavior of electromagnetic radiation
• Solve quantitative problems involving the behavior of electromagnetic radiation

Teacher Support

The learning objectives in this section will help your students master the following standards:

• (B) investigate and analyze characteristics of waves, including velocity, frequency, amplitude, and wavelength, and calculate using the relationship between wave speed, frequency, and wavelength;
• (C) compare characteristics and behaviors of transverse waves, including electromagnetic waves and the electromagnetic spectrum, and characteristics and behaviors of longitudinal waves, including sound waves; and
• (D) investigate behaviors of waves, including reflection, refraction, diffraction, interference, resonance, and the Doppler Effect.

Section Key Terms

[BL] Impress on students how incredibly fast light travels. Start with 3.00 × 10 8 m/s and express it as 300,000 km/s, or roughly the distance to the moon or the odometer reading on an old, very durable car.

[OL] Discuss fundamental physical constants. Ask students to name some others in addition to the speed of light. Supply the gravitational constant and Planck’s constant. Note that these are different from mathematical constants, such as pi.

[AL] Explain that physical constants have the same value everywhere in the universe. This realization was an important event in the history of science.

Misconception Alert

Both light and sound seem to travel very rapidly from a human perspective. Impress on students that the speed of sound is many orders of magnitude slower than the speed of light.

Types of Electromagnetic Wave Behavior

In a vacuum, all electromagnetic radiation travels at the same incredible speed of 3.00 × 10 8 m/s, which is equal to 671 million miles per hour. This is one of the fundamental physical constants. It is referred to as the speed of light and is given the symbol c . The space between celestial bodies is a near vacuum, so the light we see from the Sun, stars, and other planets has traveled here at the speed of light. Keep in mind that all EM radiation travels at this speed. All the different wavelengths of radiation that leave the Sun make the trip to Earth in the same amount of time. That trip takes 8.3 minutes. Light from the nearest star, besides the Sun, takes 4.2 years to reach Earth, and light from the nearest galaxy—a dwarf galaxy that orbits the Milky Way—travels 25,000 years on its way to Earth. You can see why we call very long distances astronomical .

When light travels through a physical medium, its speed is always less than the speed of light. For example, light travels in water at three-fourths the value of c . In air, light has a speed that is just slightly slower than in empty space: 99.97 percent of c . Diamond slows light down to just 41 percent of c . When light changes speeds at a boundary between media, it also changes direction. The greater the difference in speeds, the more the path of light bends. In other chapters, we look at this bending, called refraction , in greater detail. We introduce refraction here to help explain a phenomenon called thin-film interference .

[BL] Describe the rainbow colors that result from thin-film interference and ask students for examples. Fill in the ones they miss with soap bubbles, oil slicks, and compact discs.

[OL] Describe refraction qualitatively. Base the discussion on change of speed; there is no need to introduce refractive index yet. Give some examples from everyday experience. For example, explain why objects underwater are not exactly where they appear to be.

[AL] Explain refraction with the analogy of a four-wheeled vehicle veering from solid pavement into sand. When wheels on one side enter the sand—the slower medium—the vehicle swerves toward the sand. This is because the wheels on that side move more slowly than those on the pavement side.

Have you ever wondered about the rainbow colors you often see on soap bubbles, oil slicks, and compact discs? This occurs when light is both refracted by and reflected from a very thin film. The diagram shows the path of light through such a thin film. The symbols n 1 , n 2 , and n 3 indicate that light travels at different speeds in each of the three materials. Learn more about this topic in the chapter on diffraction and interference.

Figure 15.9 shows the result of thin film interference on the surface of soap bubbles. Because ray 2 travels a greater distance, the two rays become out of phase . That is, the crests of the two emerging waves are no longer moving together. This causes interference , which reinforces the intensity of the wavelengths of light that create the bands of color. The color bands are separated because each color has a different wavelength. Also, the thickness of the film is not uniform, and different thicknesses cause colors of different wavelengths to interfere in different places. Note that the film must be very, very thin—somewhere in the vicinity of the wavelengths of visible light.

Do not confuse polar molecules with polarized light. If a molecule is polar, it refers to a separation of negative and positive electric charges. Polarized light is light whose electric field component vibrates in a specific plane.

You have probably experienced how polarized sunglasses reduce glare from the surface of water or snow. The effect is caused by the wave nature of light. Looking back at Figure 15.2 , we see that the electric field moves in only one direction perpendicular to the direction of propagation. Light from most sources vibrates in all directions perpendicular to propagation. Light with an electric field that vibrates in only one direction is called polarized . A diagram of polarized light would look like Figure 15.2 .

Polarized glasses are an example of a polarizing filter. These glasses absorb most of the horizontal light waves and transmit the vertical waves. This cuts down glare, which is caused by horizontal waves. Figure 15.10 shows how waves traveling along a rope can be used as a model of how a polarizing filter works. The oscillations in one rope are in a vertical plane and are said to be vertically polarized. Those in the other rope are in a horizontal plane and are horizontally polarized. If a vertical slit is placed on the first rope, the waves pass through. However, a vertical slit blocks the horizontally polarized waves. For EM waves, the direction of the electric field oscillation is analogous to the disturbances on the ropes.

Light can also be polarized by reflection. Most of the light reflected from water, glass, or any highly reflective surface is polarized horizontally. Figure 15.11 shows the effect of a polarizing lens on light reflected from the surface of water.

Watch Physics

Polarization of light, linear and circular.

This video explains the polarization of light in great detail. Before viewing the video, look back at the drawing of an electromagnetic wave from the previous section. Try to visualize the two-dimensional drawing in three dimensions.

Grasp Check

How do polarized glasses reduce glare reflected from the ocean?

• They block horizontally polarized and vertically polarized light.
• They are transparent to horizontally polarized and vertically polarized light.
• They block horizontally polarized rays and are transparent to vertically polarized rays.
• They are transparent to horizontally polarized light and block vertically polarized light.

The video explains how to generate linear polarized light, and how, by uniformly changing the phase of linear polarized light, circularly polarized light can be produced.

Polarized Glasses

• EYE SAFETY—Looking at the Sun directly can cause permanent eye damage. Avoid looking directly at the Sun.
• two pairs of polarized sunglasses OR
• two lenses from one pair of polarized sunglasses
• Look through both or either polarized lens and record your observations.
• Hold the lenses, one in front of the other. Hold one lens stationary while you slowly rotate the other lens. Record your observations, including the relative angles of the lenses when you make each observation.
• Find a reflective surface on which the Sun is shining. It could be water, glass, a mirror, or any other similar smooth surface. The results will be more dramatic if the sunlight strikes the surface at a sharp angle.
• Observe the appearance of the surface with your naked eye and through one of the polarized lenses.
• Observe any changes as you slowly rotate the lens, and note the angles at which you see changes.

Explain your observations made through the polarized lenses. Describe what happened when you held the lenses together and rotated one of them. Explain the changes during rotation in terms of planes of polarization. Explain your observation of the reflective surface in terms of planes of polarization.

If you buy sunglasses in a store, how can you be sure that they are polarized?

• When one pair of sunglasses is placed in front of another and rotated in the plane of the body, the light passing through the sunglasses will be blocked at two positions due to refraction of light.
• When one pair of sunglasses is placed in front of another and rotated in the plane of the body, the light passing through the sunglasses will be blocked at two positions due to reflection of light.
• When one pair of sunglasses is placed in front of another and rotated in the plane of the body, the light passing through the sunglasses will be blocked at two positions due to the polarization of light.
• When one pair of sunglasses is placed in front of another and rotated in the plane of the body, the light passing through the sunglasses will be blocked at two positions due to the bending of light waves.

Quantitative Treatment of Electromagnetic Waves

[BL] Remember that the equation for speed is v = d / t , where v is velocity or speed, d is distance, and t is time. Rearrange the equation so that it can be used to solve for either distance or time if the other two values are known.

[OL] Ask students to recall ****the metric units of frequency and wavelength. The answer is s − 1 , s − 1 , or Hz for frequency and meters or any other metric distance unit for wavelength.

A light year is a measure of distance, not time.

We can use the speed of light, c , to carry out several simple but interesting calculations. If we know the distance to a celestial object, we can calculate how long it takes its light to reach us. Of course, we can also make the reverse calculation if we know the time it takes for the light to travel to us. For an object at a very great distance from Earth, it takes many years for its light to reach us. This means that we are looking at the object as it existed in the distant past. The object may, in fact, no longer exist. Very large distances in the universe are measured in light years . One light year is the distance that light travels in one year, which is 9.46 × 10 12 9.46 × 10 12 kilometers or 5.88 × 10 12 5.88 × 10 12 miles (…and 10 12 is a trillion!).

A useful equation involving c is

where f is frequency in Hz, and λ λ is wavelength in meters.

Worked Example

Frequency and wavelength calculation.

For example, you can calculate the frequency of yellow light with a wavelength of 6.00 × 10 − 7 6.00 × 10 − 7 m.

Rearrange the equation to solve for frequency.

Substitute the values for the speed of light and wavelength into the equation.

Manipulating exponents of 10 in a fraction can be tricky. Be sure you keep track of the + and – exponents correctly. Checking back to the diagram of the electromagnetic spectrum in the previous section shows that 10 14 is a reasonable order of magnitude for the frequency of yellow light.

The frequency of a wave is proportional to the energy the wave carries. The actual proportionality constant will be discussed in a later chapter. Since frequency is inversely proportional to wavelength, we also know that wavelength is inversely proportional to energy. Keep these relationships in mind as general rules.

[BL] Good lighting for reading is important. We will see that moving twice as far away from a light source does not give you half as much light. It decreases much more than that.

[AL] Recall the inverse-square law for gravitational force. Any force or radiation that spreads out in all directions will decrease with the square of the distance.

The rate at which light is radiated from a source is called luminous flux , P , and it is measured in lumens (lm). Energy-saving light bulbs, which provide more luminous flux for a given use of electricity, are now available. One of these bulbs is called a compact fluorescent lamp ; another is an LED (light-emitting diode) bulb. If you wanted to replace an old incandescent bulb with an energy saving bulb, you would want the new bulb to have the same brightness as the old one. To compare bulbs accurately, you would need to compare the lumens each one puts out. Comparing wattage—that is, the electric power used—would be misleading. Both wattage and lumens are stated on the packaging.

The luminous flux of a bulb might be 2,000 lm. That accounts for all the light radiated in all directions. However, what we really need to know is how much light falls on an object, such as a book, at a specific distance. The number of lumens per square meter is called illuminance , and is given in units of lux (lx). Picture a light bulb in the middle of a sphere with a 1-m radius. The total surface of the sphere equals 4π r 2 m 2 . The illuminance then is given by

What happens if the radius of the sphere is increased 2 m? The illuminance is now only one-fourth as great, because the r 2 term in the denominator is 4 instead of 1. Figure 15.12 shows how illuminance decreases with the inverse square of the distance.

Calculating Illuminance

A woman puts a new bulb in a floor lamp beside an easy chair. If the luminous flux of the bulb is rated at 2,000 lm, what is the illuminance on a book held 2.00 m from the bulb?

Choose the equation and list the knowns.

Equation: illuminance = P 4 π r 2 illuminance = P 4 π r 2

P = 2,000 lm

Substitute the known values into the equation.

illuminance = P 4 π r 2 = 2,000  lm 4 ( 3.14 ) ( 2.00 ) 2  m 2 = 39.8  lm/m 2 = 39.8  lx illuminance = P 4 π r 2 = 2,000  lm 4 ( 3.14 ) ( 2.00 ) 2  m 2 = 39.8  lm/m 2 = 39.8  lx

Try some other distances to illustrate how greatly light fades with distance from its source. For example, at 3 m the illuminance is only 17.7 lux. Parents often scold children for reading in light that is too dim. Instead of shouting, “You’ll ruin your eyes!” it might be better to explain the inverse square law of illuminance to the child.

Practice Problems

Red light has a wavelength of 7.0 × 10−7 m and a frequency of 4.3 × 1014 Hz. Use these values to calculate the speed of light in a vacuum.

• 3 × 1020 m/s
• 3 × 1015 m/s
• 3 × 1014 m/s
• 3 × 108 m/s

Check Your Understanding

Use these questions to assess student achievement of the section’s Learning Objectives. If students are struggling with a specific objective, these questions will help identify any gaps and direct students to the relevant content.

Give an example of a place where light travels at the speed of 3.00 × 10 8 m/s.

• outer space
• Earth’s atmosphere
• quartz glass

Explain in terms of distances and the speed of light why it is currently very unlikely that humans will visit planets that circle stars other than our Sun.

• The spacecrafts used for travel are very heavy and thus very slow.
• Spacecrafts do not have a constant source of energy to run them.
• If a spacecraft could attain a maximum speed equal to that of light, it would still be too slow to cover astronomical distances.
• Spacecrafts can attain a maximum speed equal to that of light, but it is difficult to locate planets around stars.

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Why is the speed of light the way it is?

It's just plain weird.

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio , and author of " How to Die in Space ." He contributed this article to Space.com's Expert Voices: Op-Ed & Insights . 

We all know and love the speed of light — 299,792,458 meters per second — but why does it have the value that it does? Why isn't it some other number? And why do we care so much about some random speed of electromagnetic waves? Why did it become such a cornerstone of physics? 

Well, it's because the speed of light is just plain weird.

Related: Constant speed of light: Einstein's special relativity survives a high-energy test

Putting light to the test

The first person to realize that light does indeed have a speed at all was an astronomer by the name of Ole Romer. In the late 1600s, he was obsessed with some strange motions of the moon Io around Jupiter. Every once in a while, the great planet would block our view of its little moon, causing an eclipse, but the timing between eclipses seemed to change over the course of the year. Either something funky was happening with the orbit of Io — which seemed suspicious — or something else was afoot.

After a couple years of observations, Romer made the connection. When we see Io get eclipsed, we're in a certain position in our own orbit around the sun. But by the next time we see another eclipse, a few days later, we're in a slightly different position, maybe closer or farther away from Jupiter than the last time. If we are farther away than the last time we saw an eclipse, then that means we have to wait a little bit of extra time to see the next one because it takes that much longer for the light to reach us, and the reverse is true if we happen to be a little bit closer to Jupiter.

The only way to explain the variations in the timing of eclipses of Io is if light has a finite speed.

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Making it mean something

Continued measurements over the course of the next few centuries solidified the measurement of the speed of light, but it wasn't until the mid-1800s when things really started to come together. That's when the physicist James Clerk Maxwell accidentally invented light.

Maxwell had been playing around with the then-poorly-understood phenomena of electricity and magnetism when he discovered a single unified picture that could explain all the disparate observations. Laying the groundwork for what we now understand to be the electromagnetic force , in those equations he discovered that changing electric fields can create magnetic fields, and vice versa. This allows waves of electricity to create waves of magnetism, which go on to make waves of electricity and back and forth and back and forth, leapfrogging over each other, capable of traveling through space.

And when he went to calculate the speed of these so-called electromagnetic waves, Maxwell got the same number that scientists had been measuring as the speed of light for centuries. Ergo, light is made of electromagnetic waves and it travels at that speed, because that is exactly how quickly waves of electricity and magnetism travel through space.

And this was all well and good until Einstein came along a few decades later and realized that the speed of light had nothing to do with light at all. With his special theory of relativity , Einstein realized the true connection between time and space, a unified fabric known as space-time. But as we all know, space is very different than time. A meter or a foot is very different than a second or a year. They appear to be two completely different things.

So how could they possibly be on the same footing?

There needed to be some sort of glue, some connection that allowed us to translate between movement in space and movement in time. In other words, we need to know how much one meter of space, for example, is worth in time. What's the exchange rate? Einstein found that there was a single constant, a certain speed, that could tell us how much space was equivalent to how much time, and vice versa.

Einstein's theories didn't say what that number was, but then he applied special relativity to the old equations of Maxwell and found that this conversion rate is exactly the speed of light.

Of course, this conversion rate, this fundamental constant that unifies space and time, doesn't know what an electromagnetic wave is, and it doesn't even really care. It's just some number, but it turns out that Maxwell had already calculated this number and discovered it without even knowing it. That's because all massless particles are able to travel at this speed, and since light is massless, it can travel at that speed. And so, the speed of light became an important cornerstone of modern physics.

But still, why that number, with that value, and not some other random number? Why did nature pick that one and no other? What's going on?

Related: The genius of Albert Einstein: his life, theories and impact on science

Making it meaningless

Well, the number doesn't really matter. It has units after all: meters per second. And in physics any number that has units attached to it can have any old value it wants, because it means you have to define what the units are. For example, in order to express the speed of light in meters per second, first you need to decide what the heck a meter is and what the heck a second is. And so the definition of the speed of light is tied up with the definitions of length and time.

In physics, we're more concerned with constants that have no units or dimensions — in other words, constants that appear in our physical theories that are just plain numbers. These appear much more fundamental, because they don't depend on any other definition. Another way of saying it is that, if we were to meet some alien civilization , we would have no way of understanding their measurement of the speed of light, but when it comes to dimensionless constants, we can all agree. They're just numbers.

One such number is known as the fine structure constant, which is a combination of the speed of light, Planck's constant , and something known as the permittivity of free space. Its value is approximately 0.007. 0.007 what? Just 0.007. Like I said, it's just a number.

So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is.

So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know.

Learn more by listening to the episode "Why is the speed of light the way it is?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com. Thanks to Robert H, Michael E., @DesRon94, Evan W., Harry A., @twdixon, Hein P., Colin E., and Lothian53 for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter.

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

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• voidpotentialenergy This is just my opinion but i think L speed is it's speed because the particle part of it is the fastest it can interact with the quanta distance in quantum fluctuation. Light is particle and wave so the wave happens in the void between quanta. Gravity probably travels in that void and why gravity seems instant. Reply
• rod The space.com article wraps up the discussion with, "So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is. So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know." It seems that the *universe* made this decision, *But our universe has chosen the fine structure constant to be...* I did not know that the universe was capable of making decisions concerning constants used in physics. E=mc^2 is a serious constant. Look at nuclear weapons development, explosive yields, and stellar evolution burn rates for p-p chain and CNO fusion rates. The report indicates why alpha (fine structure constant) is what it is and c is what it is, *We don't know*. Reply

Admin said: We all know and love the speed of light, but why does it have the value that it does? Why isn't it some other number? And why did it become such a cornerstone of physics? Why is the speed of light the way it is? : Read more

rod said: The space.com article wraps up the discussion with, "So on one hand, the speed of light can be whatever it wants to be, because it has units and we need to define the units. But on the other hand, the speed of light can't be anything other than exactly what it is, because if you were to change the speed of light, you would change the fine structure constant. But our universe has chosen the fine structure constant to be approximately 0.007, and nothing else. That is simply the universe we live in, and we get no choice about it at all. And since this is fixed and universal, the speed of light has to be exactly what it is. So why is the fine structure constant exactly the number that it is, and not something else? Good question. We don't know." It seems that the *universe* made this decision, *But our universe has chosen the fine structure constant to be...* I did not know that the universe was capable of making decisions concerning constants used in physics. E=mc^2 is a serious constant. Look at nuclear weapons development, explosive yields, and stellar evolution burn rates for p-p chain and CNO fusion rates. The report indicates why alpha (fine structure constant) is what it is and c is what it is, *We don't know*.

• rod FYI. When someone says *the universe has chosen*, I am reminded of these five lessons from a 1982 Fed. court trial. The essential characteristics of science are: It is guided by natural law; It has to be explanatory by reference to natural law; It is testable against the empirical world; Its conclusions are tentative, i.e., are not necessarily the final word; and It is falsifiable. Five important points about science. Reply
• Gary If the universe is expanding , how can the speed of light be constant ( miles per second , if each mile is getting longer ) ? Can light's velocity be constant while the universe expands ? So, with the expansion of the universe , doesn't the speed of light need to increase in order to stay at a constant velocity in miles per second ? Or, do the miles in the universe remain the same length as the universe 'adds' miles to its diameter ? Are the miles lengthening or are they simply being added / compounded ? Reply
• Gary Lets say we're in outer space and we shoot a laser through a block of glass. What causes the speed of the laser light to return to the speed it held prior to entering the block of glass ? Is there some medium in the vacuum of space that governs the speed of light ? Do the atoms in the glass push it back up to its original speed. If so, why don't those same atoms constantly push the light while it travels through the block of glass ? Reply

Gary said: Lets say we're in outer space and we shoot a laser through a block of glass. What causes the speed of the laser light to return to the speed it held prior to entering the block of glass ? Is there some medium in the vacuum of space that governs the speed of light ? Do the atoms in the glass push it back up to its original speed. If so, why don't those same atoms constantly push the light while it travels through the block of glass ?

Gary said: If the universe is expanding , how can the speed of light be constant ( miles per second , if each mile is getting longer ) ? Can light's velocity be constant while the universe expands ? So, with the expansion of the universe , doesn't the speed of light need to increase in order to stay at a constant velocity in miles per second ? Or, do the miles in the universe remain the same length as the universe 'adds' miles to its diameter ? Are the miles lengthening or are they simply being added / compounded ?

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Fundamentals of waves.

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Seeing, hearing, feeling warmth, surfing, tuning the radio, using a cellphone – these and many more activities involve waves. But what is a wave? In this article, we get to grips with the different kinds of waves and look at the key characteristics of all waves – their wavelength, period, frequency, speed and amplitude. These concepts are important for describing waves of all kinds.

Different waves, same properties

Water waves are the focus of our tsunamis and surf resources, but many other kinds of wave exist. These include sound waves, light waves, radio waves, microwaves and others. All kinds of waves have the same fundamental properties of reflection, refraction, diffraction and interference, and all waves have a wavelength, frequency, speed and amplitude.

Wave length, height and frequency

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

All waves can be thought of as a disturbance that transfers energy.

Some waves (water waves and sound waves) are formed through the vibration of particles. Waves form because water molecules are disturbed, and sound waves are formed by the disturbance of air particles or particles in an object through which sound is travelling, like a door.

Electromagnetic waves (such as light waves, UV radiation, microwaves and others) are formed through oscillating electric and magnetic fields.

Waves have a defined wavelength

Every wave has a specific wavelength. This is defined as the length from one wave crest to the next.

Different kinds of waves have widely varying wavelengths. In water, surf waves have wavelengths of 30–50 m, and tsunamis have much longer wavelengths (about 100km). Sound waves vary in wavelength according to the pitch of the sound – humans can hear sound with wavelengths between 70 mm and 70 m. Different kinds of electromagnetic waves vary greatly in wavelength, from the long wavelength of radio waves (about 10 m) to the much shorter wavelengths of visible light (less than a millionth of a metre – usually described as hundreds of nanometres) and X-rays (less than a billionth of a metre).

Wavelengths of different wave types

Every wave has a specific wavelength, but these vary widely between wave types – from hundreds of kilometres (tsunamis) to approximately a hundredth of a billionth of a metre (gamma rays).

Waves have a defined frequency

As well as a defined speed, every wave has a frequency. This is the number of wavelengths that travel past a point in 1 second of time. Frequency is measured in hertz (waves per second). For each type of wave, a longer wavelength means a lower frequency – for electromagnetic waves, for example, microwaves have a much lower frequency than UV waves.

Waves also have a defined period. This is the time it takes for the wave to undergo one complete oscillation or one complete motion. The wave period is inversely proportional to the frequency – the longer the period, the lower the frequency.

Wave speed is related to frequency and wavelength

Every wave travels at a particular speed. Water waves are unusual because waves can have different speeds – wave speed depends on how the wave is formed, which is why tsunamis travel much faster than surf waves. Unlike water waves, electromagnetic waves always travel at the same speed (3 hundred million metres per second) and sound waves all travel at the same speed in a given medium (for example, approximately 340 metres per second in air).

The speed of a wave is related to both its frequency and wavelength. The equation v = f x λ (speed = frequency x wavelength) describes this relationship and is useful for predicting the unknown characteristics of a wave.

For waves that always travel at the same speed (like electromagnetic waves), the equation can be used to work out the frequency or wavelength. High-frequency EM waves will have a short wavelength, and low-frequency waves will have longer wavelengths. For waves with a variable speed (such as water waves), the equation can be used to work out the speed from the frequency and wavelength.

All waves have an amplitude

The amplitude of a wave is normally defined as the maximum displacement of the particles within the wave from their normal equilibrium positions. For water waves, the amplitude of a wave is the distance between the wave crest and the normal water level.

People sometimes talk about the wave height, which is the vertical distance from the wave crest to the wave trough, instead of amplitude.

Nature of science

Common scientific concepts can connect seemingly disparate areas of science. For instance, water waves, sound waves and electromagnetic radiation all exhibit similar characteristics and behaviours. Some of the scientific knowledge obtained in one of these areas can therefore be applied to the others.

Related content

For a more simplified look at waves, see Waves and energy – wave basics .

Activity ideas

Use a Mexican wave to demonstrate how waves transfer energy and to help your students visualise the wave behaviours of reflection, constructive interference and shoaling.

This interactive or paper-based Venn diagram can be used to illustrate the key similarities and differences between tsunami waves and surf waves .

In the electromagnetic spectrum – picture dictation activity, students use a range of skills to investigate some ways humans use the waves on the electromagnetic spectrum.

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Understanding the Speed of Electromagnetic Waves

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Electromagnetic waves are a fundamental aspect of physics and play a crucial role in various fields, including telecommunications, astronomy, and medical imaging. These waves are responsible for the transmission of energy in the form of electric and magnetic fields. Understanding the speed at which these waves propagate is essential for comprehending their behavior and applications. In this article, we will delve into the concept of wave propagation, explore the speed of electromagnetic waves across the electromagnetic spectrum, and discuss the energy spectrum associated with these waves. We will also touch upon the implications of these concepts in real-world applications and scientific research.

Wave Propagation

Wave propagation refers to the movement of waves through a medium or space. This is a fundamental concept in physics that applies to all types of waves, including sound waves, water waves, and of course, electromagnetic waves. In the case of electromagnetic waves, they can propagate through a vacuum, such as outer space, or through a medium, such as air or water. The speed at which electromagnetic waves propagate depends on the properties of the medium through which they travel. For instance, electromagnetic waves travel slower in denser media due to the increased interaction with the particles in the medium.

Speed of Electromagnetic Waves

The speed of electromagnetic waves in a vacuum is a constant value denoted by the symbol “c.” This value is approximately 299,792,458 meters per second, which is equivalent to the speed of light. The speed of light is a fundamental constant in physics and serves as a universal speed limit. No object or information can travel faster than the speed of light in a vacuum. This concept, which is a cornerstone of Einstein’s theory of relativity, has profound implications for our understanding of space, time, and the nature of the universe.

Electromagnetic Spectrum Speed

The electromagnetic spectrum encompasses a wide range of electromagnetic waves, each with its own unique properties and applications. The speed at which electromagnetic waves propagate varies across the spectrum, but it is important to note that all electromagnetic waves travel at the speed of light in a vacuum, regardless of their frequency or wavelength.

At the lower end of the spectrum, we have radio waves. These waves have the longest wavelength and the lowest frequency. Radio waves can travel at the speed of light, just like all other electromagnetic waves. This means that radio waves also travel at approximately 299,792,458 meters per second in a vacuum. Radio waves are used in various applications, including broadcasting, radar, and satellite communication.

Moving up the spectrum, we encounter microwaves. Microwaves have shorter wavelengths and higher frequencies compared to radio waves. The speed at which microwaves propagate is also the same as the speed of light. Microwaves are used in a variety of applications, including cooking, telecommunications, and radar systems.

Next, we have infrared waves. Infrared waves have even shorter wavelengths and higher frequencies than microwaves. Again, the speed at which infrared waves propagate is equal to the speed of light. Infrared waves are used in a variety of applications, including thermal imaging, remote controls, and fiber-optic communications.

Continuing up the spectrum, we reach the visible light region. Visible light consists of different colors, each with its own wavelength and frequency. Despite the variation in wavelength and frequency, all colors of visible light travel at the same speed, which is the speed of light. Visible light is, of course, essential for vision, but it also has numerous other applications, including photography, illumination, and optical communication.

Moving further up the spectrum, we encounter ultraviolet waves. Ultraviolet waves have shorter wavelengths and higher frequencies than visible light. Similar to the previous waves, the speed at which ultraviolet waves propagate is equal to the speed of light. Ultraviolet waves are used in a variety of applications, including sterilization, air purification, and the production of vitamin D in the skin.

Next, we have X-rays. X-rays have even shorter wavelengths and higher frequencies compared to ultraviolet waves. Once again, the speed at which X-rays propagate is the same as the speed of light. X-rays are used in a variety of applications, including medical imaging, material analysis, and airport security.

Finally, at the highest end of the spectrum, we have gamma rays. Gamma rays have the shortest wavelengths and the highest frequencies of all electromagnetic waves. Just like all other electromagnetic waves, gamma rays also travel at the speed of light. Gamma rays are used in a variety of applications, including medical treatments, sterilization, and astronomical observations.

Energy Spectrum

The energy of electromagnetic waves is directly proportional to their frequency. This means that waves with higher frequencies have higher energy levels. As we move up the electromagnetic spectrum, the frequency and energy of the waves increase. This relationship between energy and frequency is described by Planck’s equation, which states that the energy of a wave is equal to its frequency multiplied by Planck’s constant.

Radio waves, at the lower end of the spectrum, have the lowest energy levels. Microwaves have slightly higher energy levels than radio waves, followed by infrared waves, visible light, ultraviolet waves, X-rays, and gamma rays, which have the highest energy levels.

The energy spectrum of electromagnetic waves is crucial in various applications. For example, in medical imaging, X-rays are used due to their ability to penetrate tissues and create detailed images of bones and internal organs. Gamma rays, on the other hand, are used in radiation therapy to treat cancer due to their high energy levels. The energy of electromagnetic waves is also crucial in telecommunications, as it determines the amount of information that can be transmitted and the distance over which it can be transmitted.

Understanding the speed of electromagnetic waves is essential for comprehending their behavior and applications. The speed of electromagnetic waves in a vacuum is a constant value known as the speed of light. This speed serves as a universal speed limit. The speed at which electromagnetic waves propagate varies across the electromagnetic spectrum, with all waves traveling at the speed of light. Additionally, the energy of electromagnetic waves increases as we move up the spectrum, with gamma rays having the highest energy levels. By understanding these concepts, we can better appreciate the role of electromagnetic waves in our everyday lives and various scientific fields. This knowledge is not only fascinating in its own right, but it also has practical implications for technology, medicine, and our understanding of the universe.

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Astronomy & Astrophysics 101: Electromagnetic Spectrum

By ESA/Hubble April 24, 2022

What Is the Electromagnetic Spectrum?

The electromagnetic spectrum is a range of frequencies of electromagnetic radiation. From long to short wavelength, the EM spectrum includes radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays.

Energy travels through space as electromagnetic (EM) waves, which are made up of oscillating electric and magnetic fields. EM waves do not require a material (such as air or water) to move through, hence they can, unlike sound, travel through empty space. All electromagnetic waves travel at the same speed in a vacuum: the speed of light (which is itself an EM wave).

An EM wave, like other waves, is defined by its wavelength, and the range of wavelengths we observe, from extremely long to very short, is referred to as the EM spectrum. The electromagnetic spectrum is loosely divided into divisions based on how the waves behave when they interact with matter, and each division is given a name. Radio waves have the longest wavelengths, followed by microwaves, infrared, visible light, ultraviolet, x-rays, and finally gamma rays, which have the shortest wavelengths. 

Celestial objects such as stars , planets , and galaxies all emit electromagnetic waves at various wavelengths, therefore telescopes are built to be sensitive to different sections of the electromagnetic spectrum. Shorter wavelengths are referred to as ‘bluer,’ while longer wavelengths are referred to as’redder.’ EM radiation in and near the visible region of the spectrum is generally referred to broadly as ‘light.’

We can construct a more complete image of an object’s structure, composition, and behavior by combining observations at different wavelengths than visible wavelengths alone can convey.

For more than three decades, Hubble has studied the Universe using its 2.4-meter primary mirror and its five science instruments. They observe primarily in the ultraviolet and visible parts of the spectrum, but also have some near- infrared capabilities. Hubble observes in different wavelength bands, one band at a time, each providing different information on the object under study. Each of these wavelengths is reproduced in a different color and these are combined to form a composite image that well resembles the true emission from that celestial object.

Astronomers have used this set of single-color images, shown around the edge, to construct the color picture (center) of a ring of star clusters surrounding the core of the galaxy NGC 1512. These pictures were taken by the NASA/ESA Hubble Space Telescope’s Faint Object Camera (FOC), Wide Field and Planetary Camera 2 (WFPC2), and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS). Each image represents a specific color or wavelength region of the spectrum, from ultraviolet to near infrared, and shows the wide wavelength region covered by Hubble. Celestial bodies emit light at a variety of wavelengths, anywhere from gamma rays to radio waves. Astronomers chose to study NGC 1512 in these colors to emphasize important details in the ring of young star clusters surrounding the core. Credit: NASA, ESA, Dan Maoz (Tel-Aviv University, Israel, and Columbia University, USA)

By exploring the image above, you can see how astronomers have used a set of single-color images to construct the color picture of a ring of star clusters surrounding the core of the galaxy NGC 1512. Each image represents a specific color or wavelength region of the spectrum, from ultraviolet to near-infrared, and shows the wide wavelength range covered by Hubble. Astronomers chose to study NGC 1512 in these colors to emphasize important details in the ring of young star clusters surrounding the core.

Electromagnetic Spectrum. Credit: ESA/Hubble

Astronomers use multi-wavelength imagery to study details that might not otherwise be present in visible images. For example, a new multiwavelength observation of Jupiter released in 2020 by Hubble in ultraviolet/visible/near-infrared light of Jupiter gave researchers an entirely new view of the giant planet. These observations provided insights into the altitude and distribution of the planet’s haze and particles and showed Jupiter’s ever-changing cloud patterns. The planet’s aurorae are only visible in the ultraviolet; however, the structure of the red spot is well studied at visible wavelengths.

To celebrate the telescope’s 25th anniversary in 2015, Hubble unveiled two new beautiful portraits of the popular Pillars of Creation , revealing how different details can be studied in visible and near-infrared observations. While the visible light captures the multi-colored glow of gas clouds, the infrared image penetrates much of the obscuring dust and gas to uncover countless newborn stars.

We invite you to watch this Hubblecast that explores how Hubble’s observations differ across different wavelengths of the electromagnetic spectrum, and how these observations will be complemented by those of the James Webb Space Telescope .

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1 comment on "astronomy & astrophysics 101: electromagnetic spectrum".

“Astronomers have used this set of single-color images, shown around the edge, to construct the color picture (center) of a ring of star clusters surrounding the core of the galaxy NGC 1512.”

Because a computer display device works with just three primary colors (RGB), I would be interested in knowing just how they compressed 7 different images into three colors. Did they use band ratios or weighted averages, or some other technique?

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2.1: Light as a Wave

• Last updated
• Save as PDF
• Page ID 94097

• Tom Weideman
• University of California, Davis

What is "Waving"?

The jump from mechanical waves to sound was a difficult one, mainly because the "displacement" of the wave changed from matter that oscillates back-and-forth, to (in the case of sound in a gas) oscillations in pressure or density. This difficulty gets greatly magnified for the case of light. We know that light is a wave based on how it behaves – it exhibits the same properties of other waves we have examined – it interferes with itself, it follows an inverse-square law for intensity (brightness), and so on. But we also know that we can see light from the sun, moon, and stars, which means that light waves can travel through the vacuum of space. Unlike every other wave we have seen, it doesn't require any medium at all! So what do we use as the "displacement" for our wave function?

Back in the 19th century, physicists studied extensively the subjects of electricity (lightning, shocking your finger on a doorknob, balloons sticking to your hair, etc.) and magnetism (compasses, sticking things to your refrigerator, etc.). It started becoming clear that the two forces, while different, had some links. Electric currents were found to affect compass needles, and magnets moving near wires were found to create electric currents. It all came together with an amazing (for the time) effort in mathematics by a man named James Clerk Maxwell. He showed that changing electric fields could induce magnetic fields, while changing magnetic fields could in turn induce electric fields. This is a recipe for propagation of these fields, and the equation he derived for this propagation was exactly the wave equation! So he predicted, from results taken from experiments in electricity and magnetism, that an electromagnetic wave could be produced. The wave equation included physical constants from both electricity and magnetism, and extracting the wave speed from this equation resulted in a number Maxwell was already familiar with – the speed of light. It is traditional to denote this speed with a lower-case 'c':

\[c = 3.0\times10^8\frac{m}{s}\]

So the "displacement" of such a wave is actually the electric and magnetic field vectors (both types of fields are waving simultaneously, with each inducing the other) in the space through which the light wave is traveling. Don't worry that this doesn't make much sense right now – it should be a bit clearer when you get to Physics 9C and study electricity & magnetism.

Okay, so for light we now have the wave speed and the "displacement." Let's address a couple other elements of light as a wave. First, a medium is not needed, as electric and magnetic field can exist in a vacuum. The presence of a medium (such as air or water) does effect the electric and magnetic fields, because media are made up of atoms, which are composed of positive and negative electric charges. Because of this, the speed of light within a medium is different (slower) than its speed in a vacuum. Mathematics and experiments show that light is a transverse wave – the electric and magnetic field vectors point in directions that are perpendicular to the direction of motion of the light wave (and as it turns out, they also rare always perpendicular to each other).

Figure 2.1.1 – Electromagnetic Wave

The red arrows in the figure above represent electric field vectors, and blue arrows magnetic field vectors. Specifically, this is a plane-polarized EM wave, which means the field vectors of a given type remain in a single plane. We will discuss plane polarization soon, but it should be noted that EM waves do not have to behave this way, so long as the electric and magnetic field vectors remain perpendicular to each other and to the direction of motion. For example, a circularly polarized EM wave features electric and magnetic field vectors that circulate their directions (while remaining perpendicular to each other and the direction of motion) as the wave propagates, like the hands of an analog clock, and can do so in a clockwise or counterclockwise manner.

Finally, we need to say two things about light perception. For sound, intensity (proportional to amplitude-squared) is perceived as loudness, and for light it is brightness. For sound, frequency is perceived as pitch, and for visible light it is perceived as color. The qualification "visible" must be appended because we can only see a very limits spectrum of light frequencies, the rainbow of colors often described with the acronym ROYGBIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet). The red end of the visible spectrum exhibit the lowest frequencies, and the violet the highest. But of course light waves can come in frequencies much lower and much higher, and at various arbitrary cutoffs, they are given names you have probably heard before. In order of increasing frequency below the red end of the visible spectrum we have: radio waves , microwaves , and infrared ; and above the violet end of the spectrum: ultraviolet , x-rays , and gamma rays .

Huygens's Principle

When we discussed the case of a wave on a string, we said that the wave causes each particle on the string to vibrate up-and-down in harmonic motion. It should therefore not be surprising that if we grab the string at a single point and force it to vibrate in harmonic motion, that a wave will propagate away from that point. In fact, this gives us a way of describing how the wave propagates: The wave causes a single point to oscillate, which in turn causes a wave to be generated, which then vibrates another point, and so on. In the 17th century a Dutch scientist named Christian Huygens generalized this idea to three dimensions. The principle which now bears his name can be stated this way:

Every (3-dimensional) wave propagates by having every point on a wavefront being an independent generator of a new spherical wave, and the interference of all of those individual spherical waves results in the overall wave observed.

When we look at a single point light source, the farther away it is, the flatter the light wavefronts will be when they reach us. When the source is very far away (e.g. the sun), then the wavefronts are essentially flat. We call waves with such flat wavefronts plane waves , for obvious reasons. But now the question arises, “If Huygens’s principle is valid, how can plane waves occur?” After all, each point on the plane wave behaves as a point source of a spherical wave. Let's look at the spherical wave contributions of many point sources on a plane. We'll do this gradually, starting with just a few points on a plane, and filling in the spaces between them little-by-little:

Figure 2.1.2 – Plane Wave from Huygens's Principle

One might ask why a plane wave only propagates in a single direction. Suppose a plane wave propagating to the right. If each new wavefront becomes a source for a new wave, why don't waves come out of it in both directions? It is difficult to express in a simple diagram like the one above the effects of superposition, but the short answer is that there is destructive interference between all of the previous wavefronts and the new one, which results in zero wave energy traveling "backwards."

It should also be noted that a plane wave is a one-dimensional wave, which means that its intensity does not drop off with distance. But the intensities of the spherical wavelets do follow an inverse-square law. So if they get weaker with distance, why don't plane waves? The reason is that the farther a wavelet travels, the more other wavelets it encounters. These encounters result in constructive interference, bolstering the amplitude (and therefore the intensity) The rate at which the wavelets encounter other wavelets and constructively interfere is exactly enough to compensate for each wavelet losing its own individual intensity, maintaining the plane wave's intensity.

Where Huygens's principle becomes particularly useful is in explaining what happens when a plane wave encounters a barrier. A plane wave moves straight ahead because there is destructive interference of the wavelets in other directions. But a barrier removes a number of wavelets by either absorbing or reflecting the part of the wavefront from which those wavelets were going to spawn. The result is that the wave "bends around corners," a phenomenon known as diffraction .

Figure 2.1.3 – Diffraction from Huygens's Principle

Like other wave phenomena, this is not unique to light. Ocean waves diffract around barriers like reefs, peninsulas, and docks. It's certainly possible to hear a sound made from around a corner. It should be noted that the part of a wave that diffracts around a corner is no longer a plane wave, and is subject to the reduction in intensity the farther it travels. Of course reflections of waves are also responsible for their ability to change direction in the presence of barriers, but the phenomenon of diffraction in conjunction with interference leads to other important observable properties that we will deal with next.

You should be aware that diffraction is so intimately tied up with the interference effects that it causes (the subjects of the next few sections) that many physicists use the word "diffraction" to indicate the interference phenomena themselves, rather than the "going around corners" definition.