# Physics-Light and Sound (Science)

"The most important fundamental laws and facts of physical science have all been discovered, and these are now so firmly established that the possibility of their ever being supplemented in consequence of new discoveries is exceedingly remote." -Albert A. Michelson (1852-1931)

The science of physics includes the study of waves. You know about waves from watching the ocean or the surface of a lake or pond. Other types of waves include sound and electromagnetic radiation, such as light, radio waves, microwaves, and x-rays. An earthquake occurs when waves traveling below the surface cause the ground to move up and down or side to side.

Waves can be described mathematically, and the same equations can be used for all kinds of waves. But it is not necessary to understand the equations to observe how waves function. The key thing to know about a wave is that it transfers energy from one place to another. Electromagnetic waves can carry energy through space. Electromagnetic waves include the light that you see; radio waves that carry information to your television, cordless phone, and car radio; x-rays that make an image as they pass through your body; and the microwaves with which you cook dinner. These waves are all related and they all carry energy.

Mechanical waves include sound, water waves, and earthquakes. Mechanical waves don’t travel through space; they travel through a medium such as air, water, or Earth itself. The energy of mechanical waves is carried by the motion of the particles of the medium. If you watch "the Wave" course through the stands of a large football stadium, you can observe the energy of each individual particle as the fans carry their energy around the stadium.

## Why is the sky blue?

When astronauts in the International Space Station look out the window, they don’t see a yellow sun in a blue sky. Instead, they see a white sun against a black background. So why does the sky look blue from where you stand on the ground? And for that matter, why do the puffy cumulus clouds floating across the blue sky appear to be white (except when the sun is rising and setting)?

Light travels in waves of different lengths. The wavelengths of visible light are very short—about one ten-thousandth to one one-thousandth of a millimeter. The longest waves in the familiar spectrum of visible light are red and the shortest are violet. The sun produces light across the entire span of the spectrum. When all of the wavelengths of the visible spectrum are seen at once, the light appears white. That’s why the sun looks white to an observer on the space station. The sun also produces ultraviolet radiation, which has a shorter wavelength than that of violet light, and infrared radiation, which has a wavelength longer than red light. However, since you can’t detect these types of radiation with your eyes, they don’t concern us in this discussion.

The wavelength of a wave is the distance between one crest of a wave and the next crest. The length of a wave of red light may be 640 billionths of a meter while the length of an ocean wave may be many meters.

When light reaches Earth, it has to pass through the atmosphere, the layer of gases that surrounds the solid and liquid parts of the planet. The atmosphere is full of dust particles and droplets of water vapor. When light hits these particles, it reflects and changes direction. As the light passes through the atmosphere, it is reflected over and over. That’s why sunlight can reach places that are not in a direct line to the sun. Water droplets are larger than the light waves so they tend to reflect every wavelength. That explains why a cloud looks white—the droplets reflect all colors of light and the combination looks white.

Most of the atmosphere consists of molecules of nitrogen and oxygen, which are much smaller than the wavelength of light. When light strikes these molecules, it does not reflect in the same way that it reflects from water droplets. However, when light strikes them, the molecules occasionally absorb the energy of the light wave. After a while (a very tiny fraction of a second, generally), the molecules emit light at the same wavelength. While all of the light from the sun is traveling in the same direction, the light emitted by a gas molecule can travel in any direction in a straight line away from the molecule. The light is scattered in all directions. This is called Rayleigh scattering, named after Lord John Rayleigh, who first described it.

So how does this explain the color of the sky? It turns out that the shorter the wavelength of the light, the more likely it is to be scattered by Rayleigh scattering. Long wavelengths, such as red and yellow, are much less likely to be scattered than short wavelengths, such as violet and blue. If there were no atmosphere the sky would look black, just as it does in space. What you see when you look at a blue sky is blue sunlight that has bounced around in the air until it is coming at you from all directions.

Why, then, doesn’t the sky look violet? After all, violet light has an even shorter wavelength than blue light. There are two reasons for this. First, the sun emits more blue light than it does violet light, so the scattered blue light tends to be more visible to the eye because there is more of it. Second, your eye is much more sensitive to blue light that it is to violet light. These effects combine to make the sky look blue.

The sky often looks red in the early morning and late evening. During these times, sunlight must travel a greater distance through the atmosphere to reach the observer. Shorter wavelengths are scattered along the way, so the light that reaches your eye is the long-wavelength red light.

## Why does a spoon in a glass of water seem to bend?

Place a spoon in a clear glass that is half full of water. Look down at the spoon from different angles and you will see that its stem seems to bend or break where it enters the water. You can pull the spoon out and see that it is not bent, so what happened? The apparent bending occurs because light does not always travel at the same speed.

"Wait a minute," you say, "I always heard that light travels at a constant speed. Wasn’t that the basic assumption of Einstein’s theories?" Well, Einstein based his theory on the speed of light in a vacuum, which is constant at about 186,000 miles per second. When light passes through a transparent material, such as air, water, or glass, it does not travel at the same speed as in a vacuum. Light moves through materials at a slower speed than it moves through a vacuum.

In addition, the speed of light in the material depends on the nature of the material itself. For most purposes, the speed of light in air can be considered to be the same as the speed of light in a vacuum. In water, however, there is a large difference. Light travels through water at about three fourths the speed at which it travels through a vacuum. The speed of light in glass depends on the composition of the glass, but generally it is about two thirds the speed of light in a vacuum.

The apparent bend in the spoon’s stem comes from this change in light speed as the light travels between the water and the air. You see the spoon because rays of light are reflected from its surface. When these rays of light pass from the water into the air, they travel at a different speed.

When parallel rays of light travel through the water, some reach the interface first and begin to move faster. Others take longer to reach the air, so their change in speed is delayed longer. As a result, the path that the light follows is bent. This bending of the light path causes the spoon to appear to bend. If you go fishing and see a large fish in the water, you cast your bait in front of the place where the fish appears to be swimming. Because of refraction, the fish seems farther away than it actually is.

The refractive index of a material is the ratio of the speed of light in a vacuum divided by the speed of light in the material. The refractive index determines the angle of refraction as light enters and leaves the material.

The amount of bending depends on the change in light’s speed as it passes from one material to another. The angle of change when light passes from air into a transparent material is called the refractive index of the material.

We use the change in the angle of light whenever we wear corrective lenses. If the lens of your eye does not focus an image at the right place on the retina, the object that you are looking at appears blurry. Eyeglasses use lenses that are carefully shaped to bend light as it enters and leaves the glass to change the point where light rays come together. The shape of the lens determines whether the image moves forward or backward in the eye. The correct lens places a sharp image on the retina.

Diamond has one of the highest refractive indices of natural materials. The speed of light in a diamond is only about 40 percent of the speed of light in air. The large refraction in diamond, combined with reflections from the cut facets, give the diamond its characteristic sparkle.

## Why does the pavement ahead look wet on a hot, dry day?

What comes to mind when you hear the word "mirage"? If you grew up watching cartoons, you probably picture a thirsty, haggard man crawling across the desert, looking at an oasis, complete with palm trees and a shimmering lake. Is it a figment of a mind driven by thirst or something else? Many people think of a mirage as a delusion created by stress, thirst, and exhaustion. While the mind may add an illusion to it, a mirage is an actual phenomenon of light that can be recorded by a camera.

Because of their popular portrayal, it may seem that a mirage is specifically a hot desert phenomenon. Actually, a mirage can occur under any conditions in which a temperature gradient can bend light. Mirages can even be seen above the ice in Antarctica.

You can often see a mirage as you drive along the highway on a hot day. Watch the road far ahead and you see puddles that disappear as you get closer. No matter how fast or how far you drive, you never reach the puddle. The mirage is caused by the same trick of light as the bent spoon-refraction. The speed of light in air changes slightly as the temperature changes.

Mirages occur when a sharp boundary forms between two layers of air with different temperatures. If the air just above the ground is much warmer than the air above it, light bends upward. The puddle that you see on the road ahead is actually light from the blue sky and clouds in the distance. Your mind, however, treats the image as if it occurred in a straight line from your eyes, not accounting for the bending of the light. It appears as though the light from the sky is reflecting from the surface of a puddle or lake. This type of mirage is usually seen on hot days when the sun heats the air above the pavement, but it can occur whenever the air near the ground is much warmer than the air above it. Sometimes you can see images of objects other than the sky in a refracted image. Try looking at the air above a dark-colored car that has been sitting in the sun on a hot day. You may see an inverted image of the storefront on the other side of the street.

There is another type of mirage that occurs when cold air lies beneath warmer air. Then light can bend downward so that a distant object appears above its actual position. When this happens, an object such as a ship on the ocean can be seen even though it is beyond the horizon. Sometimes mountains or buildings can be seen floating above the ground.

The combination of refraction and reflection in the atmosphere can create amazing mirages that appear to float in the sky. The fate Morgan is one of the most amazing mirages, as distant mountains or icebergs appear to be castles and towers floating in the air above the open waters of the sea.

## What makes a rainbow move as you drive?

If there is a pot of gold at the end of the rainbow, you would expect to have a better chance of reaching it in a car than on foot. Unfortunately, the rainbow will fade before you reach it, no matter how you travel. The physics of light guarantees that no one will ever arrive at the point where the rainbow touches the ground.

To understand how a rainbow forms, consider what happens when sunlight passes through a prism. As the light crosses the boundary between air and glass, it is refracted, or bent so that it travels at a different angle. When the light leaves the glass, it is refracted again. The prism shows the colors of the spectrum because different colors of light are bent by different amounts. Although all light travels at the same speed in a vacuum, the speed of light in matter depends on its wavelength. Because the angle at which light is refracted is a function of the change in speed, each wavelength bends by a slightly different amount than any other wavelength. The result is the familiar separation of white sunlight into its component colors.

Rain consists of many spherical drops of water (not the teardrop shape that is often drawn for raindrops). When sunlight passes from air into a drop of water, the light bends. Some rays of the light reflect from the back of the drop. When these rays pass from water to air, they bend again. Each wavelength (color) of light bends at its own characteristic angle and comes out of the drop at a different angle. A ray of red light

"Since the rainbow is a special distribution of colors (produced in a particular way) with reference to a definite point—the eye of the observer—and as no single distribution can be the same for two separate points, it follows that two observers do not, and cannot, see the same rainbow." that enters the drop can reflect and leave at an angle of 42° compared to the entry ray. A ray of blue light exits at an angle of 40°. If you are standing in the path of one of the rays that exits the drop, you will see a light of that color. If there are many drops of water, as in a rainstorm or a fine mist from a hose, the light you see from each drop depends on your position compared to that drop. You see the sum of these light rays as a rainbow.

There are a couple of implications of this explanation of rainbows. First, a rainbow will only form when the sun is behind you. Otherwise there is no light ray to refract and reflect back to your eye. Second, you can never reach the rainbow, no matter how quickly you move toward it. The rainbow is an image formed by water drops in your line of sight. As you approach the drops, you see them from a different angle and no longer detect the light rays of the rainbow. However, light from more distant drops may still reach you, so you find that the rainbow recedes as you move.

What happens if light does not leave the drop after being reflected? It can reflect from the front of the drop to the back and reflect again toward the front. When this happens, the ray leaves the drop at a greater angle and you see a double rainbow. The outer rainbow is not as bright and its colors are reversed.

## Why do colored objects look different under different lighting?

Have you ever looked at a paint sample in the store and then taken it home only to find that it looks completely different? The color of the sample shouldn’t change during the drive home, so what happened?

How you perceive the color of an object depends on two different factors: the wavelengths of light that are coming from the object to your eyes, and how your eyes and brain interpret those wavelengths. Some objects—for example a light bulb, television, or electric heating element—emit light. Other objects, such as a wall, a leaf, or this topic, reflect light from other sources.

"The main source of all technological achievements is the divine curiosity and playful drive of the tinkering and thoughtful researcher, as much as it is the creative imagination of the inventor."

When the emitted or reflected light reaches your eye, it interacts with cells in your eye. There are four types of cells. Three types of cone cells are sensitive to different ranges of light wavelengths. These cone cells used to be called blue, green, and red cones but they are actually sensitive to a wide range of wavelengths centering around these colors.

There is a great deal of overlap in the wavelengths that stimulate the cells, so a single wavelength may be detected to different degrees by two or even three types of cone cells. That’s where the brain comes in. Your brain converts the combined signals from the cone cells into the perception of a particular color. It does not matter whether the signals come from a single wavelength or a combination of many wavelengths. A particular set of stimulations is perceived as a particular color. That explains why a computer monitor can produce all possible colors using a combination of three colors of dots. A fourth type of cell in the eye, the rod cell, does not distinguish color. Rods generally work only when the level of light is low. Cone cells do not work well in low light, so you tend to see in black and white when it is dark.

So why do objects seem to have different colors in different types of lighting? Picture a green leaf on a sunny day. Why does the leaf look green? When sunlight strikes the leaf, molecules inside the leaf absorb some of the energy of the light. Some wavelengths are absorbed more than others. This is the source of energy that a plant needs in order to grow. The wavelengths of sunlight that are not absorbed by the leaf are reflected. When you look at a leaf, you see the light that is reflected from its surface and you perceive this light as green. What happens if the light that strikes the leaf is not sunlight, but rather light from a red stage light that produces only light in red wavelengths? In this case, the leaf absorbs all of the light that strikes it. When no light is reflected or emitted, an object appears to be black.

There are several kinds of artificial light used in buildings. Although the light they produce generally appears to be white, none of these light sources produce a spectrum of light that exactly matches sunlight in distribution and relative intensity of wavelengths. They also differ from one another. For example, incandescent light (light bulbs) tends to produce more light in red and yellow wavelengths compared to sunlight. Standard fluorescent tubes tend to produce light that has a greater proportion of violet and blue wavelengths. The light that is reflected from an object varies depending on the wavelengths of light that strike it. That is why a paint chip may appear different under different types of lighting. Some stores that sell paint offer a place where the colors can be observed under varying types of light.

Animals do not necessarily see things the same way that you do. Some birds and fish have four types of cone cells, so their color vision may be more precise than ours. Owls, however, like many other nocturnal animals, have no cone cells. While they have excellent vision, they perceive the world in black and white. Bug zappers use ultraviolet lights to attract mosquitoes because they see the short-wavelength violet and ultraviolet light, but not yellow or orange. And you know how a matador uses a red cape to entice a bull to charge? Well, the color is for the spectators. Bulls have only two kinds of cone cells and they do not perceive red as a color.

## Why do you see lightning before you hear thunder?

During a thunderstorm, you see a flash of lightning on a nearby hilltop. Several seconds later, you hear a loud clap of thunder. Which was produced first—the lightning or the thunder, or did they occur at the same time? Most people today know that the thunder and lightning occur at the same time, but that has not always been clear. In fact, about 2,300 years ago, Aristotle thought that thunder occurred when trapped air was forced out of clouds. Lightning was thought to follow the thunder when this released air burned. So, lightning occurred after thunder and it only appeared to come first because we see it first. Later, many people found it obvious that the lightning occurred first because we see it first.

There is a common belief that lightning does not strike the same place twice. There is no scientific basis for this belief. Lightning strikes the highest points of very tall buildings, such as the Empire State Building, dozens of times every year. If you watch closely, lightning sometimes seems to flicker several times. These flickers are repeated strikes on one spot within about a second.

We now know that thunder and lightning occur at about the same time. Lightning is an incredibly hot electrical discharge that occurs when opposite charges build up inside clouds or between clouds and the ground. The air around the discharge is heated almost instantaneously to about 50,000°F (five times the temperature of the sun’s surface). When the air is heated so quickly, a violent expansion creates a shock wave in the air around it, much like that of an explosion. This wave travels through the air, carrying vibrations to your ears, where they are perceived as the sound of thunder.

So why do you see the lightning first? Light travels through air at about 186,000 miles every second. Sound, however, is much slower. A sound wave travels through the air as vibrating molecules bump into their neighbors, transferring energy. This process takes time, so a sound wave moves about 1,000 feet in that same second (the speed varies a bit, depending on temperature and the amount of water vapor in the air). You can estimate the distance to the lightning strike based on this difference in the speed of light and sound. Count the seconds between the time you see the lightning and the time you hear the thunder. Divide the number of seconds by 5 to find the approximate distance, in miles, to the lightning strike.

You may have noticed that sometimes thunder sounds like a sharp clap and other times it is a dull rumble. This difference can also be explained by the time that it takes for sound to reach your ears. Because the electrical discharge is essentially instantaneous along the length of the lightning bolt, the sound wave is created instantaneously as well. If the lightning strike is very close to your position, the sound waves do not spread very far before reaching your ear. Then you hear a loud crack or bang. However, if the lightning strike is far away, sound waves from the top of the lightning bolt may travel much farther than those at the bottom. Because the sound wave may be several miles across, you hear a long rumble of thunder.

On average, there are about 6,000 flashes of lightning (and peals of thunder) every minute around the planet. Thunder and lightning always occur together but you may occasionally detect one without the other. The sound of thunder can travel about 10 miles, but the light from a lightning strike may travel much farther, especially if it reflects off the water vapor in clouds, so you see the flash but hear no thunder. In other circumstances, the flash may be hidden by dense clouds so you hear distant thunder without seeing the lightning.

## What causes an echo and why can’t you hear one in your living room?

If you have ever been in a wide canyon with distant walls, or even a large empty room such as a gymnasium with solid walls, you have probably heard an echo. A shout is repeated, sometimes more than once. What causes an echo and why can’t you hear one in your living room?

A sound wave travels through air as a series of compressions of the gases that it travels through. What happens when these compressions strike a flat object such as a wall? If the wall is soft and absorbent, such as the tapestry or cork coverings on the walls of a concert hall, at least part of the energy of the wave is absorbed. If the wall is smooth and hard, like the rock walls of a canyon or the concrete walls of a large gym, the waves may reflect just as light waves reflect from a mirror. An echo occurs when sound waves reflect and return to your ear.

An echo occurs whenever a sound wave reflects. However, if the echo occurs less than one tenth of a second after the original sound, you generally cannot separate the two sounds, so you don’t hear the echo. At normal room temperature, sound travels in air at about 750 mph (or 340 meters per second). That means you will normally only hear an echo when sound bounces off an object that is at least 17 meters away. This will be the case in a large gymnasium but not in the typical living room.

In the early years of the twentieth century, it was widely reported that a duck’s quack does not echo. Researchers at the University of Salford, England, tested this idea by placing a duck named Daisy in a chamber designed to measure the reflections of sound. They determined that a quack, like any other sound, does indeed reflect, causing an echo. They proposed three possible explanations for the origin of the belief: 1. the quack is usually too quiet for its echo to be detected; 2. ducks don’t quack near reflecting surfaces; 3. it is hard to hear the echo of a sound that fades in and fades out.

You can use the echo to determine the distance between you and a mountain or a canyon wall. Make a loud, sharp sound and measure the time until you hear the echo. Knowing that sound travels at 340 meters per second, you can find the distance in meters by multiplying the number of seconds by 340 and then dividing by 2 (remember that the sound has to travel to the wall and then the echo must return the same distance).

Echoes allow bats to fly very quickly in the dark without colliding with walls, light poles, or other objects. Bats make very high-pitched sounds, too high for humans to detect. If you look at pictures of bats, you will see that they generally have large ears, the better for hearing echoes. Using the time between the sound and the echo, a bat can accurately determine the location of an object and turn to avoid it. Fortunately for bats, they can determine the echo in times far less than 0.1 seconds.

## Why does the sound of a racecar engine change as it passes?

In the stands of a stock car race, or even on a television broadcast of the race, you can hear a change in the sound of the car as it passes. As the car approaches, the powerful engine makes a high-pitched whine. Suddenly, as it passes your observation point, the engine sound drops to a low rumble. Why would the position of the car affect the sound of its engine?

The Doppler effect, named for Austrian scientist Christopher Doppler, who described it in 1842, is a change in the frequency of a wave due to motion of the source relative to the observer. You can detect the Doppler effect whenever an object that makes a constant sound is moving relative to your position. Examples include the sound of a whistle on a passing train, a siren on a police car, and even a mosquito flying by.

You can picture the Doppler effect by thinking of sound waves at a specific wavelength as compressions in the air that occur at a constant interval. As each compression enters your ear, it causes sensors to vibrate and send a message to your brain that is interpreted as sound. Shorter Frequency is the measure of the number of waves passing a point in a unit of time. Frequency is often measured in hertz (Hz), or waves per second. The frequency of visible light ranges from 450 trillion Hz to 750 trillion Hz. Human ears can detect sound in a frequency range of 20 Hz to 20,000 Hz. Ocean waves typically occur at frequencies in the range of 0.05 Hz to 1 Hz. wavelengths have a greater frequency—that is, compressions occur at a greater rate. A higher frequency corresponds to a higher pitch. If the source of the sound is moving toward you, each compression starts at a point that is closer to you than the previous compression. As a result, succeeding waves reach your ear more frequently than they are emitted by the source, so the sound has a higher pitch. As the source moves away from you, the waves are spread out and reach you with less frequency. The pitch of the sound is then lower. The same thing would happen if the source of the sound were stationary and you were moving quickly toward or away from it.

The Doppler effect applies to all kinds of waves, not just sound. For example, when a police officer uses radar to measure your speed, that is an application of the Doppler effect. The radar system sends radio waves at a constant frequency toward your car.

The wave that is reflected back is shorter if you are moving toward the detector. By comparing the frequency of the returning wave to the frequency of the original wave, a computer in the radar detector can calculate your speed. Other applications of radar include tracking the motion of airplanes and ships, and even monitoring a thunderstorm by measuring the motion of water droplets in a cloud. Astronomers even use the Doppler effect to measure the expansion of the universe. The light from distant galaxies is shifted toward the red end of the spectrum because the distance between them and our galaxy is increasing at rates close to the speed of light.

The Doppler effect can be used in medicine. Doctors use ultrasound, sound waves that have a frequency higher than those detected by the human ear, to measure the speed of blood flowing inside the body. The waves reflect off the moving fluid and return to the detector. The change in frequency indicates the speed of the blood flow.

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