As I discussed previously, the sky is blue because tiny particles in the atmosphere (dust, clumps of air molecules, microscopic water droplets) are better at deflecting shorter wavelength blue light than they are at deflecting longer wavelength red light. As sunlight passes through the atmosphere, enough blue light is deflected (or more technically Rayleigh scattered) by these particles to give the atmosphere an overall blue glow. The sun itself is slightly reddened by this process because a fraction of its blue light is deflected away before it reaches our eyes.

But at sunrise and sunset, sunlight enters our atmosphere at a shallow angle and travels a long distance before reaching our eyes. During this long passage, most of the blue light is deflected away and virtually all that we see coming to us from the sun is its red and orange wavelengths. The missing blue light illuminates the skies far to our east during sunrise and to our west during sunset. When the loss of blue light is extreme enough, as it is after a volcanic eruption, so little blue light may reach your location at times that even the sky itself appears deep red. The particles in air aren't good at deflecting red wavelengths, but if that's all the light there is they will give the sky a dim, red glow.

The foam consists of tiny air bubbles surrounded by very thin films of soap and water. When light enters the foam, it experiences partial reflections from every film surface it enters or exits. That is because light undergoes a partial reflection whenever it changes speed (hence the reflections from windows) and the speed of light in soapy water is about 30% less than the speed of light in air. Although only about 4% of the light reflects at each entry or exit surface, the foam contains so many films that very little light makes it through unscathed. Instead, virtually all of the light reflects from film surfaces and often does so repeatedly. Since the surfaces are curved, there is no one special direction for the reflections and the reflected light is scattered everywhere. And while an individual soap film may exhibit colors because of interference between reflections from its two surfaces, these interference effects average away to nothing in the dense foam. Overall, the foam appears white--it scatters light evenly, without any preference for a particular color or direction. White reflections appear whenever light encounters a dense collection of unoriented transparent particles (e.g. sugar, salt, clouds, sand, and the white pigment particles in paint).

As for the fact that even colored soaps create only white foam, that's related to the amount of dye in the soaps. It doesn't take much dye to give bulk soap its color. Since light often travels deep into a solid or liquid soap before reflecting back to our eyes, even a modest amount of dye will selectively absorb enough light to color the reflection. But the foam reflects light so effectively with so little soap that the light doesn't encounter much dye before leaving the lather. The reflection remains white. To produce a colored foam, you would have to add so much dye to the soap that you'd probably end up with colored hands as well.

Most objects make no light of their own and are visible only because they reflect some of the light that strikes them. Without sunlight (or any other light source), these passive objects would appear black. Black is what we "see" when there is no light reaching our eyes from a particular direction. The only objects we would see would be those that made their own light and sent it toward our eyes.

The fact that we see mostly reflected light makes for some interesting experiments. A red object selectively reflects only red light; a blue object reflects only blue light; a green object reflects only green light. But what happens if you illuminate a red object with only blue light? The answer is that the object appears black! Since it is only able to reflect red light, the blue light that illuminates it is absorbed and nothing comes out for us to see. That's why lighting is so important to art. As you change the illumination in an art gallery, you change the variety of lighting colors that are available for reflection. Even the change from incandescent lighting to fluorescent lighting can dramatically change the look of a painting or a person's face. That's why some makeup mirrors have dual illumination: incandescent and fluorescent.

The one exception to this rule that objects only reflect the light that strikes them is fluorescent objects. These objects absorb the light that strikes them and then emit new light at new colors. For example, most fluorescent cards or pens will absorb blue light and then emit green, orange, or red light. Try exposing a mixture of artwork and fluorescent objects to blue light. The artwork will appear blue and black: blue wherever the art is blue and black wherever the art is either red, green, or black. But the fluorescent objects will display a richer variety of colors because those objects can synthesize their own light colors.

Ozone is an unstable molecule that consists of three oxygen atoms rather than then usual two. Because of its added complexity, an ozone molecule can interact with a broader range of light wavelengths and has the wonderful ability to absorb harmful ultraviolet light. The presence of ozone molecules in our upper atmosphere makes life on earth possible.

However, because ozone molecules are chemically unstable, they can be depleted by contaminants in the air. Ozone molecules react with many other molecules or molecular fragments, making ozone useful as a bleach and a disinfectant. Molecules containing chlorine atoms are particularly destructive of ozone because a single chlorine atom can facilitate the destruction of many ozone molecules through a chlorine recycling process.

In contrast, nitrogen molecules are extremely stable. They are so stable that there are only a few biological systems that are capable of separating the two nitrogen atoms in a nitrogen molecule in order to create organic nitrogen compounds. Without these nitrogen-fixing organisms, life wouldn't exist here. Because nitrogen molecules are nearly unbreakable, they survive virtually any amount or type of chemical contamination.

When a light wave passes through matter, the charged particles in that matter do respond--the light wave contains an electric field that pushes on electrically charged particles. But how a particular charged particle responds to the light wave depends on the frequency of the light wave and on the quantum states available to the charged particle. While the charged particle will begin to vibrate back and forth at the light wave's frequency and will begin to take energy from the light wave, the charged particle can only retain this energy permanently if doing so will promote it to another permanent quantum state. Since light energy comes in discrete quanta known as photons and the energy of a photon depends on the light's frequency, it's quite possible that the charged particle will be unable to absorb the light permanently. In that case, the charged particle will soon reemit the light.

In effect, the charged particle "plays" with the photon of light, trying to see if it can absorb that photon. As it plays, the charged particle begins to shift into a new quantum state--a "virtual" state. This virtual state may or may not be permanently allowed. If it is, it's called a real state and the charged particle may remain in it indefinitely. In that case, the charged particle can truly absorb the photon and may never reemit it at all. But if the virtual state turns out not to be a permanently allowed quantum state, the charged particle can't remain in it long and must quickly return to its original state. In doing so, this charged particle reemits the photon it was playing with. The closer the photon is to one that it can absorb permanently, meaning the closer the virtual quantum state is to one of the real quantum states, the longer the charged particle can play with the photon before recognizing that it must give the photon up.

A colored material is one in which the charged particles can permanently absorb certain photons of visible light. Because this material only absorbs certain photons of light, it separates the components of white light and gives that material a colored appearance.

A transparent material is one in which the charged particles can't permanently absorb any photons of visible light. While these charged particles all try to absorb the visible light photons, they find that there are no permanent quantum states available to them when they do. Instead, they play with the photons briefly and then let them continue on their way. This playing process slows the light down. In general blue light slows down more than red light in a transparent material because blue light photons contain more energy than red light photons. The charged particles in the transparent material do have real permanent states available to them, but to reach those states, the charged particles would have to absorb high-energy photons of ultraviolet light. While blue photons don't have as much energy as ultraviolet photons, they have more energy than red photons do. As a result, the charged particles in a transparent material can play with a blue photon longer than they can play with a red photon--the virtual state produced by a blue photon is closer to the real states than is the virtual state produced by a red photon. Because of this effect, the speed at which blue light passes through a transparent material is significantly less than the speed at which red light passes through that material.

Finally, about quantum states: you can think of the real states of one of these charged particles the way you think about the possible pitches of a guitar string. While you can jiggle the guitar string back and forth at any frequency you like with your fingers, it will only vibrate naturally at certain specific frequencies. You can hear these frequencies by plucking the string. If you whistle at the string and choose one of these specific frequencies for your pitch, you can set the string vibrating. In effect, the string is absorbing the sound wave from your whistle. But if you whistle at some other frequency, the string will only play briefly with your sound wave and then send it on its way. The string playing with your sound waves is just like a charged particle in a transparent material playing with a light wave. The physics of these two situations is remarkably similar.

Apparently there are conditions in which green light from the sun is bent by the atmosphere so that it is visible first as the sun begins to rise above the horizon. Instead of seeing the yellow edge of the sun peaking up from behind the water or land, you see a green edge that lasts a second or two before being replaced by the usual yellow. This green flash is the result of refraction (bending of light) and dispersion (color-dependent light-speed) in air and is discussed in considerable detail at http://www.isc.tamu.edu/~astro/research/sandiego.html. According to the author of that site, Andrew Young, given a low enough horizon, which is the primary consideration, and clear air, which is also important, and a little optical aid, which helps a lot, one can certainly see green flashes at most sunsets.

When light passes into a material, it interacts primarily with the negatively charged electrons in that material. Since light consists in part of electric fields and electric fields push on charged particles, light pushes on electrons. If the electrons in a material can't move long distances and can't shift from one quantum state to another as the result of the light forces, then all that will happen to the light as it passes through the material is that it will be delayed and possibly redirected. But if the electrons in the material can move long distance or shift between states, then there is the chance that the light will be absorbed by the material and that the light energy will become some other type of energy inside the material.

Which of these possibilities occurs in a particular organic material depends on the precise structure of that material. Carbon atoms can be part of transparent organic materials, such as sugar, or of opaque organic materials, such as asphalt. The carbon atoms and their neighbors determine the behaviors of their electrons and these electrons in turn determine the optical properties of the materials.

A rainbow is truly circular, not parabolic. Passing through the exact center of that circle is the line that runs between the sun and your head. Each colored arc of the rainbow is located at a particular angle away from this line--the red arc is farther from the line than the violet arc is.

The glass in the rear view mirror is cut so that it forms a thin wedge--it's thicker at the top than it is at the bottom. Its back surface is fully mirrored by a layer of aluminum. For daytime use, the mirror is oriented so that light from behind the car enters the glass, reflects from the layer of aluminum on the back surface, and returns through the glass to your eyes.

But when you tip the mirror upward for night use, the mirrored back surface presents you only with a view of the car's darkened ceiling. However, there is a weak second reflection from the clear front surface of the mirror--whenever light changes speeds, as it does upon entering the glass, some of that light reflects. About 4% of the light striking the front surface of the mirror from behind the car reflects without entering the glass and is directed toward your eyes. Since the image you see is about 25 times dimmer than normal, it doesn't blind you the way a reflection from the mirrored surface would.

Light consists of electromagnetic waves, meaning that it is composed of electric and magnetic fields. While light isn't affected by other electric or magnetic fields, it is affected by gravitational fields. Like everything else in our universe, light falls when exposed to gravity. However, because light travels so fast, it's very hard to detect that it falls. The first observation of light falling in a gravitational field was made during a total eclipse in 1919 and served as dramatic confirmation of the predictions of Einstein's general theory of relativity. As for light traveling in a circle, this can occur near the surface of a black hole. When light traveling tangent to the surface of the black hole falls at just the right rate, it will orbit the black hole indefinitely.

The first step in explaining a mirage is to understand why the sky is blue, or why it has any color at all. If it weren't for the earth's atmosphere, the sky would be black and dotted with stars. That's how the moon's sky appears. But the earth's atmosphere deflects some of the sunlight that passes through it, particularly short-wavelength light such as blue and violet, and this scattered light (Rayleigh scattering) gives the sky its bluish cast. When you look at the blue sky, you're seeing particles of light that have been scattered away from their original paths into new paths so that they reach your eyes from all directions.

The blue light from the sky normally travels directly toward your eyes so that you see it coming from the sky. But when there is a layer of very hot air near the ground in the distance, some of the blue light from the sky in front of you bends upward toward your eyes. This light was traveling toward the ground in front of you at a very shallow angle but it didn't hit the ground. Instead, its entry into the hot air layer bent it upward so that it arced away from the ground and toward your eyes. When you look at the ground far in front of you, you see this deflected light from the blue sky turned up at you by the air and it looks as though it has reflected from a layer of water in front of you. This bending of light that occurs when light goes from higher-density cold air to lower-density hot air is called refraction, the same effect that bends light as light enters a camera lens or a raindrop or a glass of water. Whenever light changes speeds, it can experience refraction and light speeds up in going from cold air to hot air. In this case, the light bends upward, missing the ground and eventually reaching your eyes.

Sonar stands for "sound navigation ranging" and involves the bouncing of sound waves from objects to determine where those objects are. It's based on the reflection of sound waves from objects. Whenever a wave of any sort moves from one medium to another and experiences a change in speed (or more generally, a change in impedance), part of that wave reflects. Because sound travels much faster in solids than it does in air, some sound reflects when it moves from air to rock--which is why you hear echoes when you yell at a mountain! But even more subtle changes in the speed of sound will cause modest reflections. Thus a sophisticated sound generator and receiver can detect objects immersed in water or buried in the ground. Another form of sonar is used in medical imaging--ultrasonic imaging.

When a light wave enters matter, the light wave's electric field causes charged particles in the matter to accelerate back and forth. That's because an electric field exerts forces on charged particles. The light wave gives up some of its energy to these charged particles and is partially absorbed in the process. However, the charged particles don't retain the light's energy very long. They are accelerating and accelerating charged particles emit electromagnetic waves. In fact, they reemit the very same light wave that they absorbed moments earlier. Overall, the light wave is partially absorbed and then reemitted by each electrically charged particle it encounters, so that the light continues on its way as though nothing had happened.

However, something has happened--the light wave has been delayed ever so slightly. This absorption and reemission process holds the light wave back so that it travels at less than its full speed. If the charged particles in the matter are few and far between, this slowing effect is almost insignificant. But in dense materials such as glass or diamond, the light wave can be slowed substantially.

Actually, higher frequency violet light is slowed more than lower frequency red light because violet light is more effectively absorbed and reemitted by the atoms in most transparent materials. That's because when a high frequency light wave encounters the electrons in an atom, the jiggling motion is so rapid and the electrons' motions are so small that the electrons never reach the boundaries of the atom. As a result, those electrons are able to jiggle back and forth as though they were free electrons and they do a good job of slowing the light wave down. But when a low frequency light wave encounters the electrons in an atom, the jiggling motion is slower and the electrons' motions are so large that they quickly reach the boundaries of the atom. As a result, those electrons aren't able to jiggle back and forth as far as they should and they don't slow the light wave down as well.

Infrared, visible, and ultraviolet light are all electromagnetic waves. However these waves differ in both their wavelengths (the distances between adjacent maximums in their electric fields) and in their frequencies (the number of electric field maximums that pass by a specific point in space each second). Infrared light has longer wavelengths and lower frequencies than visible light, while ultraviolet light has shorter wavelengths and higher frequencies than visible light. We can't see infrared or ultraviolet lights because the cells of retinas aren't sensitive to these lights. Nonetheless, we can often tell when those lights are present--we may feel infrared light as heat on our skins and we may find ourselves sunburned by ultraviolet light.

A rainbow isn't an image that originates at a specific distance away from your eyes. It consists of rays of colored light that travel at particular angles away from the water droplets that produce them. You see red light coming toward you from a certain angle because at that angle, the water droplets are all sending red light toward you. In the garden hose case, the water droplets are so densely arranged that they are able to create a brilliant rainbow in only a few meters of thickness. In a typical rainstorm, sunlight must travel through hundreds or thousands of meters of raindrops to produce an intense rainbow. When you look up toward the red arc of the normal rainbow, you are seeing light directed toward your eyes by millions of water droplets, some close and others distant, that are all sending a part of the red portion of the sunlight striking them toward you and the other wavelengths of sunlight elsewhere.

You are correct that a normal rainbow is cut off abruptly by the horizon and that it would continue down below to form a full circle if the ground weren't in the way. People in airplanes sometimes see full 360° rainbows.

Biological tissues themselves are relatively transparent. They're not good conductors of electricity and electric insulators are typically transparent (quartz, diamond, sapphire, salt, sugar). But we also contain some pigment molecules that are highly absorbing of certain wavelengths of light. For example, the hemoglobin molecules in blood absorb green and blue light quite strongly, so that they appear red. When you look at a flashlight through your hand, the light appears red because of this absorption of green and blue light by hemoglobin. If you use a bright enough red light source and are willing to look very carefully, probably with sophisticated light sensing devices, you can probably see a little light coming through a person's body. But that light will probably have bounced several times during its passage, so that you won't be able to learn anything about what the person's internal organs look like. To get a better view of what a person's insides look like, you need light that penetrates more effectively and that doesn't bounce very often. Moreover, you must employ techniques to that block this bouncing light as much as possible so that you only see light that travels straight through the person. The light that does this isn't visible light--it's X-rays. X-rays are very high frequency, very short wavelength "light" (or rather electromagnetic waves). Tissue doesn't absorb these X-rays much at all and they can go through people to form images.

I would expect that certain black light sources would cause tanning with only modest burning while other black light sources would cause burning with only modest tanning. Black light--also known as ultraviolet light--consists of very energetic light particles. The particles or photons of ultraviolet light contain enough energy to break chemical bonds and rearrange molecules. When you're exposed to such energetic light, it causes damage to molecules in your skin cells and your skin may respond by darkening in the process we call "tanning." But ultraviolet light is a general term that covers a broad range of wavelengths and photon energies. Long wavelength/low energy ultraviolet light tends to cause tanning while short wavelength/high energy ultraviolet light tends to cause burning--it directly kills cells. But these differences aren't sharp and any ultraviolet light will cause some amount of skin damage.

Because light is an electromagnetic wave, it is emitted and absorbed by electric charges. For an electric charge to emit light it must move--in fact, the charge must accelerate. For an electric charge to absorb light it must also move--it must also accelerate. However, there are many materials that do not have mobile electric charges. For example, while all electric insulators have electric charges in them, those electric charges can't move long distances. The electric charges in many electric insulators can't even move enough to absorb light and the light simply passes right through them. They are transparent.

Although I am not certain, I would guess that most automatic foghorns detect the fog optically. They either send light from a source to a detector and turn on the foghorn when the detector fails to see the light or they send light into their surroundings and turn the foghorn on when they see excessive reflection of that light.

While the expression "blue moon" usually refers to the infrequent occurrence of second full moon in a calendar month, there have been rare occasions when the moon truly appeared blue. In those cases, an unusual fire or volcanic eruption filled the air with tiny clear particles that had just the right sizes to resonantly scatter away the red portion of the visible light spectrum so that only bluish light from the moon was able to pass directly to the viewer's eyes. The moon thus appeared blue.

Red sunsets are much more common and they are caused by Rayleigh scattering--the non-resonant scattering of light by particles that are much smaller than the light's wavelength. While Rayleigh scattering is rather weak, it's weaker for long wavelength light (red light) than it is for short wavelength light (violet light). As a result, blue and violet lights are scattered more than red light; making the sky appear blue and the sun and moon appear red, particularly when they are low on the horizon and most of their blue light is scattered away before it reaches your eyes. When there is extra dust in the air, such as after a volcanic eruption, Rayleigh scattering is enhanced and the red sunsets are particularly intense.

Sunlight provides virtually all the energy in our world. Without it, plants wouldn't grow and we wouldn't have food or daylight. We wouldn't even have fossil fuels such as coal and petroleum because those were formed from vegetation that itself derived energy from the sun. However, sunlight also contains ultraviolet light, which can damage chemicals in biological tissue. Long exposure to ultraviolet light can age your skin or cause cancer.

Although our eyes are insensitive to 1000 nanometer infrared light, there are two ways to detect it effectively. The easiest is to use an inexpensive black-and-white surveillance video camera. Many of these cameras are sensitive to a broader spectrum of light than are our eyes and they can see 1000 nanometer light. If you check around, you should be able to find one that sees the light you're interested in. The other technique is to use a phosphorescent or "glow in the dark" material. When exposed to visible light, the atoms in such a material become trapped in electronic states that can emit visible light only after a very long random wait. But exposing a phosphorescent material to infrared light can shift the states of the atoms in the material to new states that can emit light immediately. Thus exposing some phosphorescent materials to infrared light causes them to emit light promptly. You can then see these materials glow particularly brightly after storing visible light energy in them and then exposing them to infrared light. However, they'll only glow briefly before you have to "recharge" them by exposing them to more visible light.

Like all waves, ultrasound reflects whenever it passes from one material to another and experiences a change in speed (or more accurately, a change in impedance). Any inhomogeneity in a metal is likely to change the speed of sound in that metal and will cause some amount of sound reflection. With the proper instruments emitting sound and detecting the reflected sound, it's possible to image the imperfections. The same technique is used in medical ultrasound to image organs or fetuses, and even to image the insides of the earth.

While light travels as electromagnetic waves, it's emitted and absorbed as particles called "photons." Each photon carries with it a tiny bit of energy. The amount of energy in a photon depends on the wavelength of the light associated with it. While a photon of red light contains too little energy to cause chemical processes to occur in most molecules, a particle of violet or ultraviolet light contains enough energy to cause significant chemical damage to a typical molecule. Since sunlight contains a substantial amount of violet and ultraviolet lights, it can cause a fair amount of chemistry to occur in the molecules that absorb it. That's why colors often fade in sunlight. Many colored molecules are relatively fragile and are damaged by photons of ultraviolet light. The portion of a dye molecule that gives it its color is called a "chromophore" and is usually the most fragile part of the molecule. Destroying its chromophore will often leave a dye molecule colorless. Exposure to sunlight was the traditional way to bleach fabrics and make them white.

As you looked around, you would see a general glow of radio waves, microwaves, and infrared light coming from every surface. That's because objects near room temperature emit thermal energy as these long-wavelength forms of light. While we don't normally see such thermal radiation unless an object is hot enough for some of it to be in the visible range, your new vision would allow you to see everything glow. The warmer an object is, the brighter its emission and the shorter the wavelengths of that emission. People would glow particularly brightly because of their warm skin.

You would also see special sources of radio waves, microwaves, and infrared light. Radio antennas, cellular telephones, and microwave communication dishes would be dazzlingly bright and infrared remote controls would light up when you pressed their buttons.

You would see ultraviolet light in sunlight and from the black lights in dance halls. But there wouldn't be much other ultraviolet light around to see, particularly indoors. X-rays and gamma rays would be rare and you might only see them if you walked into a hospital or a dentist's office. Gamma rays would be even rarer, visible mostly in hospitals.

Like most liquid crystal displays (LCD), these devices use liquid crystals to alter the polarization of light and determine how much of that light will emerge from each point on the display. Liquid crystals are large molecules that orient themselves spontaneously within a liquid--much the way toothpicks tend to orient themselves parallel to one another when you pour them into box. The liquid crystals used in an LCD display are sensitive to electric fields so that their orientations and their optical properties can be affected electronically. The liquid crystals in the display occupy a thin layer between transparent electrodes and two polarizing plastic sheets. Light from a fluorescent lamp passes through a polarizing sheet, an electrode, the liquid crystal layer, another electrode, and another polarizing sheet. The orientation of the liquid crystal determines whether light from the first polarizing sheet will be able to pass through the second polarizing sheet. When electric charges are placed on the two electrodes, the liquid crystal's orientation changes and so does light's ability to pass through the pair of polarizing sheets.

To create a full color image, the display has many rows of electrodes on each side of the liquid crystals and a pattern of colored filters added to the sandwich. In "active" displays, there are also thin-film transistors that aid in the placement of charges on the electrodes. Overall, the display is able to select the electric charges on each side of every spot or "pixel" on the screen and can thus control the brightness of every pixel.

When white light strikes a molecule, that molecule may absorb some of the light. Light interacts with molecules as particles called "photons" and whether a particular photon is absorbed depends on the structure of the molecule and the color of the photon. Each molecule has the ability to absorb only certain colors of light. For example, a particular molecule may absorb only red photons. As a result, your eye will see only green and blue light photons coming from that molecule when it's exposed to white light and you will perceive that molecule as having a blue-green color known as cyan. In general, the colors that you see coming from molecules that are illuminated by white light are the colors of light that the molecules don't absorb.

The colder the air is, the less humidity it can hold. That's because at low temperature, water molecules in the air are much more likely to land on a surface and stick than they are to break free from a surface and enter the air. Thus cold air is relatively free of water molecules. Water molecules in the air tend to bind together briefly and form tiny particles that scatter light. The sky is blue because of such scattering from tiny particles. With less water in the air, there is less scattering of sunlight. As a result, the sky is a darker blue, almost black, and the sunlight that reaches you directly from the sun retains a larger fraction of its blue light. The sun appears less red and more blue-white than on a warmer, more humid day.

There are many possible methods for measuring the speed of light, but the classic technique is easiest to describe. In this method, a rapidly spinning mirror is used to direct a beam of light down a long pipe toward a stationary mirror at the end of that pipe. The first mirror is spinning in such a way that the beam it reflects sweeps across the pipe and can only strike the second mirror during that brief moment when the first mirror is perfectly aligned to direct the light down the pipe. A scientist then looks into the spinning mirror to observe the flash of light that returns from the second mirror. Because it takes a small but finite amount of time for the light to travel back and forth through the pipe, the spinning mirror will have turned a little between the moment when it sent the beam of light toward the far mirror and the moment when that beam of light returns to the spinning mirror. By studying the angle at which the reflected beam leaves the spinning mirror and by knowing how quickly the mirror is spinning, the scientist can determine the speed of light.

However, something has changed since those sorts of measurements were done: the speed of light is now a defined constant. It isn't measured any more--it's simply defined to be 299,792,458 meters per second. The second is defined in a similar manner--as 9,192,631,770 periods of a particular microwave emission from the cesium-133 atom. Because of these two definitions, an experiment that "measures the speed of light" is now used to determine the length of the meter.

Light consists of electromagnetic waves. An electromagnetic wave is a self-sustaining disturbance in the electric and magnetic fields that can exist even in empty space. You have probably seen two electrically charged objects push or pull on one another, such as when a sock clings to a shirt as you pull the two from the clothes dryer. You have probably also seen two magnetically poled objects push or pull on one another, such as when a magnet pulls itself toward a refrigerator door. These electric and magnetic forces are mediated by electric and magnetic fields respectively and, while those fields certainly exist in the space between the sock and shirt or between the magnet and refrigerator, they can also exist all by themselves. In an electromagnetic wave, the electric field creates the magnetic field and the magnetic field creates the electric field so that these two fields go on creating one another indefinitely as the wave travels through space at an enormous speed--the speed of light. Electromagnetic waves are distinguished by their frequencies or wavelengths, characteristics that are familiar to anyone who has watched water waves approaching the beach. But only a certain group of electromagnetic waves are visible to our eyes--those with frequencies between about 4.0*10¹⁴ cycles per second and 7.5*10¹⁴ cycles per second (wavelengths between about 750 nanometers and 400 nanometers). Outside of this range are infrared light at the low frequency end and ultraviolet light at the high frequency end.

A rainbow is caused by three important optical effects: reflection, refraction, and dispersion, all working together. The rainbow forms when sunlight passes over your head and illuminates falling raindrops in the sky in front of you. This sunlight enters each spherical raindrop, partially reflects from the back surfaces of the raindrop, and then leaves the raindrop and heads toward you. The raindrop helps some of the sunlight make a near U-turn. But the path that the light follows after it enters the raindrop depends on its color. Light bends or "refracts" as it changes speed upon entering water from air and the amount it bends depends on how much its speed changes. Since violet light slows more than red light, a phenomenon called "dispersion," the violet light bends more than the red light and the two colors begin to follow different paths through the drop. All the other colors are spread out between these two extremes.

The colored rays of light then partially reflect from the back surface of the raindrop because any change in light's speed also causes partial reflection. Now the various colors are on their way back toward you and the sun. The light bends again as it emerges from the raindrop and the various colors leave it traveling in different directions. Only one color of light will be aimed properly to reach your eyes. But there are other raindrops above and below it that will also send light backward and some of that light will also reach your eyes. But this light will be a different color. What you see when you observe the rainbow is the lights that many different raindrops send back toward your eyes. The upper raindrops send their red light toward your eyes while the lower raindrops send their violet light toward your eyes. You see a series of colored bows from these different raindrops.

White light is a mixture of various light waves with different wavelengths and thus different colors. When white light hits an object, some of the light waves are absorbed while others are not. The light that isn't absorbed may pass through the object or it may be reflected in a new direction. The light that you observe coming from the object is this transmitted or reflected light. If the light that you see doesn't include the same mixture of wavelengths that first hit the object, you won't see this light as white. Instead, you'll see it as colored. If the light you see contains mostly long wavelengths of light, you'll see it as red. If the light contains mostly short wavelengths of light, you'll see it as blue or violet. The wide range of colors that objects have comes from subtle differences in the wavelengths of light they absorb. However, when an object is illuminated with colored light, the light that it transmits or reflects may be altered. After all, it can't transmit or reflect a light wave that never hit it in the first place. Even variations in "white" light can affect an object's color--makeup looks different in incandescent "white" light than it does in fluorescent "white" light because those illuminations contain different mixtures of light waves.

As it passes through the atmosphere, sunlight can be deflected by a process known as Rayleigh scattering. When sunlight passes through any material, its light waves cause electric charges in the material to jiggle back and forth. That's because light waves contain electric fields and electric fields exert forces on electric charges. When the charges in a material jiggle back and forth, they may emit light. In this case, the material can absorb the sunlight for an instant and reemit it in a new direction. This process, whereby jiggling electric charges in a material absorb a light wave and reemit it in a new direction, is Rayleigh scattering.

Rayleigh scattering is extremely inefficient in particles that are much smaller than the wavelength of the light, so that visible light can travel through miles of molecules in the atmosphere before it experiences significant Rayleigh scattering. But blue light has a shorter wavelength than red light and thus experiences Rayleigh scattering more often than red light. As a result, the atmosphere tends to send the blue portion of sunlight off in every direction. Thus when you look at the atmosphere, it appears blue.

Actually, the polarizing material you are referring to is a plastic that has been impregnated with iodine atoms. The plastic, polyvinyl alcohol, is heated and stretched to align its long molecules in a particular direction. This plastic is then exposed to iodine, which binds to the long molecules and forms the equivalent of molecular wires along the direction of the aligned plastic molecules. These molecular wires absorb light that is polarized along them because the light's electric field points along its polarization direction and pushes electric charges wastefully along the iodine wires. This light is absorbed and its energy is converted to thermal energy, leaving only light with the other polarization.

When light enters a material such as glass, the light slows down. That's because the electric charges in the material delay a light wave by interacting with the wave's electric and magnetic fields. The higher the frequency of the light wave, the more it interacts with the charges in most materials and the more that light wave slows down. Thus high-frequency violet light slows more than low-frequency red light as the two enter a piece of glass.

Because of this slowing effect, light bends when it encounters a glass surface at an angle. The wave has a width and as it encounters the glass surface, one side of the wave reaches the glass before the other side of the wave. Since the side that arrives first also slows first, the whole wave bends so that it travels more directly into the glass. Since violet light slows more than red light, the violet light also bends more than the red light. The two colors thus follow different paths through the glass.

The same bending occurs in reverse when the light leaves the glass. Light speeds up as it leaves glass and again the violet light bends more than the red light. In a prism (or any carefully cut glass, crystal, or plastic), the colors of light bend differently at each surface and follow slightly different paths both in and out of the prism. The light rays then appear separately when they strike a surface outside the prism or when you look at those light rays with your eyes.

To start with, light slows down when it moves from air to ice and speeds up when it moves from ice to air. That's because the electric charges in matter can delay a light wave and slow it down. Since electric charges are more concentrated in ice than they are in air, light travels more slowly in ice than it does in air. Next, some light reflects whenever light changes speed. That's why a glass windowpane reflects some light from both its front and back surfaces. Similarly, light reflects from each surface of an ice crystal. Finally, snow is a jumbled heap of ice crystals. These clear crystals partially reflect light in all different directions like billions of tiny mirrors. The result is a white appearance. You see this exact same effect when you look at white sand, granulated salt, granulated sugar, clouds, fog, white paint, and so on. Each of these materials consists of tiny, clear objects that partially reflect light in all directions. Since they reflect all colors of light equally and spread that light in all direction equally, they appear white.

When the snow melts and becomes water, it stops having all those tiny surfaces to partially reflect light. Instead, it has only its top surface and this surface continues to partially reflect light. That's why you see reflections in the top of a puddle.

There are so many answers to these questions that I'll have to pick and choose. For their similarities, I'll note that they're both disturbances that travel through space and that both have wavelengths and frequencies. Sound is a pressure disturbance in the air (or in another material) and consists of compressions and rarefactions that travel outward from their origin. The distance between adjacent regions of compression (or rarefaction) is the sound's wavelength and the number of compressed regions that pass by a particular point each second is the sound's frequency (or pitch). Light is an electromagnetic disturbance in space itself, although materials that are present in that space can alter its characteristics somewhat. It consists of electric and magnetic fields that travel outward as waves from their origin. The distance between adjacent regions of maximum electric field (or magnetic field) in one direction is the light's wavelength and the number of regions in which the electric field points maximally in a particular direction that pass by a particular point each second is the light's frequency (or color). I hope that you can see some of the similarities in these descriptions.

As for differences, sound is a longitudinal wave--meaning that the air involved in the pressure fluctuations moves back and forth in the direction of the wave's travel. Thus if sound is moving from left to right, the air is also fluctuating back and forth from left to right. In contrast, light is a transverse wave--meaning, that the electric and magnetic fields involved in the wave fluctuate back and forth at right angles to the direction of the wave's travel. Thus if light is moving from left to right, the electric and magnetic fields associated with it are fluctuating either up and down or toward you and away from you (or both). Another difference is that sound travels about 300 meters per second and its speed depends on the speed of the air through which it travels. Light, on the other hand, travels about 300,000 kilometers per second and its speed in vacuum (empty space) is absolutely constant. The speed of light is one of the fundamental constants of the universe.

It depends on the shape of the prism and the angle at which the light arrived at the prism. Whenever light's speed changes as it passes through a surface at an angle, the light bends. Since light travels faster in air than in glass (or plastic), it bends when it goes from air to glass or from glass to air. When light enters glass, it slows down and it bends toward the normal to the surface (toward the line that's at right angles to the surface). When light leaves glass, it speeds up and it bends away from the normal to the surface. To know exactly how far the light bends, you need to know how much the glass slows light (the glass's refractive index) and the angle at which the light encountered the glass surface (the angle of incidence). You can then use one of the basic laws of optics, Snell's law, to determine the angle at which the light continues through the glass. You can then do the same for the light's emergence from the glass and determine the angle at which it leaves.

Sunscreens contain pigments that absorb invisible ultraviolet radiation. While they appear clear and transmit visible light so that you can't see them when they're on your skin, sunscreens are almost opaque to ultraviolet light. A sunscreen's SPF is related to the fraction of ultraviolet light that it absorbs. An SPF of 15 means that a normal layer of this sunscreen on your skin transmits only 1 part in 15 of the ultraviolet light that reaches it from the sun. An SPF of 40 means that a layer of this sunscreen transmits only 1 part in 40 of the ultraviolet light. The true transmission of the sunscreen depends somewhat on how you apply it and how much you apply, so these SPF ratings are only approximate. A sunscreen contains a mixture of dye molecules that transmit visible light but absorb ultraviolet light (and convert its the light's energy into thermal energy). Most if not all of these dye molecules are artificial organic compounds that have been carefully selected to be non-toxic and non-irritating. The first popular sunscreen contained a compound called PABA that caused skin reactions in many people, but more recent dye choices are less likely to cause skin trouble.

The earth's oceans and sky both appear blue to everyone who observes them. They do this because water absorbs blue light less strongly than it absorbs other colors. When ocean water is exposed to sunlight (white light), it absorbs most of the red light quickly and a good fraction of the green light. But the blue light penetrates to considerable depth in the water and there is a reasonable chance that this light will be scattered back upward to an observer on the shore, in the air, or even on the moon.

Light has precisely zero mass and that makes all the difference. You're right that taking a massive particle up to the speed of light is impossible because doing so would, in a certain sense, give the particle an infinite mass. But the more important issue here is that doing so would require an infinite amount of energy and momentum.

Most physicists use the word mass to mean a particle's mass at rest--its rest mass--and as you bring the particle to higher and higher speeds, its rest mass doesn't change. However, the relationship between the particle's energy and its momentum does change with speed and the particle's momentum begins to increase more rapidly than it should according to the older, pre-relativistic mechanical theories. In an effort to explain this anomalous increase in momentum while retaining the old Newtonian laws of motion, people sometimes assign a fictitious "mass" to the particle; one that equals the rest mass when the particle is stationary but that increases as the particle's speed increases. As a particle approaches the speed of light, its momentum increases without limit and so does its "mass." Not surprisingly, the limitless rises in energy, momentum, and "mass" prevent the massive particle from ever reaching the speed of light.

As for light, it really does have zero mass and therefore can't be described by the Newtonian laws of motion. All light has is its momentum and its energy. In fact, light can't travel slower than the speed of light because that would require it to have a mass! So the world of particles is divided into two groups: massless particles that must travel at the speed of light and massive particles that can never travel at the speed of light.

The two techniques you mention, dry ice and ultrasound, are both intended to make tiny droplets of water in the air, effectively producing an artificial cloud. While I can't think of any better ways to make such water droplets, I can think of ways to make fogs of other materials. Tiny particles of any clear material will work because what you are seeing is the random scattering of light as it's partially reflected from the front and back surfaces of clear particles. I'd suggest a chemical process that produces tiny clear particles. The easiest one I can think of is to place a dish of household ammonia (ammonium hydroxide--ammonia gas dissolved in water) and a dish of hydrochloric acid (hydrogen chloride gas dissolved in water, sold as muriatic acid by hardware stores) in your enclosed area. The two gases will diffuse throughout your enclosure and react to form tiny clear particles of ammonium chloride. The enclosure will fill with a dense white fog. The particles are so small, that they will remain in the air for a very long time, but they will eventually settle on surfaces and leave a white powdery residue. So, unlike a water fog, this chemical fog is a little messy. You shouldn't breathe the fog, either.

The moon orbits the earth from west to east. By that, I mean that if the earth were to stop turning, the moon would then rise in the west and set in the east. During a total solar eclipse, the moon is drifting directly in front of the sun. Since the moon moves from west to east, it will first block the western edge of the sun, the western limb. In contrast, during a total lunar eclipse, the moon is drifting into the earth's shadow. Since it is moving from west to east, its eastern edge will enter the shadow first.

A white shirt doesn't absorb visible light (or at least very much visible light), but it may absorb lots of infrared light. Since much of the sun's light and heat are in the form of invisible infrared light, that infrared absorption can be very important. There are many materials that appear white to your eye that do absorb strongly in the infrared and thus get very hot in sunlight.

During the day, the sky is blue because the air and dust in the air scatter mainly blue light toward your eyes. They also scatter some red light, but the blue light dominates. But at sunset, things change. The setting sun approaches the earth's atmosphere at a very shallow angle so that it must travel many kilometers through the air before reaching your eyes. During this long trip, most of the blue light is scattered away and the sun appears very red. If the path is long enough, the blue light is scattered away many kilometers to your west so that there isn't much of it left. When this occurs, even the sky around you appears somewhat reddish because there just isn't any more blue to scatter. The missing blue light is visible to people living 50 or 100 kilometers to the west as their blue sky.

Your eye works very hard to keep all of the different wavelengths of light together so that they can form sharp images on your retina without any color errors. If you look at a white light bulb, all of the different colors from that bulb must arrive together on your retina or else you will see colors where they shouldn't be. Keeping these colors together is no small task and is one of the biggest problems encountered by lens makers for cameras and telescopes. The chlorine in a pool evidently upsets your eye's ability to control these color errors. However, I'm not sure what goes wrong or why chlorine causes this problem.

The problem with looking at the sun during a solar eclipse is not that it is somehow brighter than normal but rather that (1) you tend to stare at it and (2) the size of its bright region is reduced so that it doesn't hurt as much to stare at it. It's hard to stare at the full sun because it feels uncomfortable but looking at a tiny part of the sun may not feel bad enough to make you avert your eyes. Nonetheless, that tiny part of the sun can cook your retina and cause permanent damage.

The colors of flames can be deceiving because they involve emissions from particular atoms (which impart their own characteristic colors to the light they emit). However, a blue-hot object such as a star is hotter than a red-hot object such as a glowing coal in the fireplace.

Actually, 96% of the light hitting the "other side of a water droplet" does pass out of the droplet. What you see in the rainbow is the 4% that reflects back from the far side of the water droplet. If all of the light reflected, the rainbow would be much brighter.

Purple (or violet) light travels slower in most materials than does red light. That occurs because violet light is higher in frequency than red light and gives the charged particles that it jiggles about less time to move up and down. With very little time to move, these charged particles barely notice that they are parts of atoms and molecules and respond easily to the passing electromagnetic wave. But when red light pushes and pulls on charged particles, there is more time for them to find the limits of their freedom. These charged particles are not able to move so easily when pushed on by a passing wave of red light so they do not interact with that passing wave as well as with one of violet light. Thus red light passes by with less effect and it behaves more like it would in empty space. Violet light, which interacts relatively strongly with the atoms it passes, slows down more than red light. Since red light travels more quickly than violet light, it bends less in passing through a prism. Violet light slows down more and bends more than red light.

On a sunny day, heat from the pavement can create a layer of very hot air at the surface of the road. Since hot air is less dense than cold air, its index of refraction is slightly less than that of cold air, too. As light from the sky enters this layer of low-index air, that light is bent. Light from the sky far out in front of you is turned upward so that you see the sky "reflected" from the road's surface (actually bent upward by the air above the road's surface). You interpret this sky light as coming from a pool of water on the road. But as you approach the road and look down at it, you see that the road is dry and black.

It is most often caused by the bending of light by mist around the light or by flaws in the optical components through which you are viewing the light. Whenever light passes through a clear material, its path bends. In most cases, you only notice that the light is distorted by its passage through the material. But different colors (wavelengths) of light bend by slightly different amounts so that the colors of light sometimes appear to come from slightly different directions. That's the origin of the rainbow you see.

Although I do not really know very much about the connection between sunspots and radio reception, I believe that the problem lies in with the solar wind. The solar wind is a steady stream of electrically charged particles that is responsible for the aurora, among other things. Since charged particles that interact with the earth's magnetic field accelerate, they emit radio waves. These waves should cause reception problems on earth. If anyone reading this knows otherwise or has more information, please let me know.

Fine mists of water are basically spherical water droplets in air and these can produce rainbows in exactly the same manner as raindrops do in natural rainbows.

Polarizing sunglasses block half the light (stopping horizontally polarized light and passing only vertically polarized light). But sunglasses of all types contain chemicals that absorb light of both polarizations. The darkness of the sunglasses depends on which chemicals are used and how much of those chemicals they contain. Some sunglasses are also coated with thin metallic layers that reflect a fraction of the light that strikes them. These semi-transparent mirrors can change the transmission of the sunglasses dramatically so that those sunglasses may transmit 50% of the light or 0.01% of the light. The manufacturer can choose.

Dark fabrics or surfaces are very good at absorbing and emitting light. That is why they are dark. They must contain electric charges that move fairly easily (making them good antennas) and these charges must be good at exchanging energy with the surrounding material as heat. When light strikes these charges, the charges begin to move and absorb the light's energy. This energy flows into the material as heat. Since the light is absorbed, the material appears dark (no light is reflected back toward you). But the material will also emit light very effectively when hot. If you heat a black object up, heat will flow into the charges, which will begin to move and will emit light. Thus black objects are good at both absorbing and emitting light.

Brown water contains colored contaminants that provide the color. Brown is the typical end result for a random mixture of pigments. The blue ocean is caused mostly by the sky. Since the ocean reflects some of the light from the sky, it appears blue. Pure water is almost completely colorless. Thus a glass of water has no color (unless you illuminate it with colored light). But if you look at a white light through many meters of water, that light will become slightly colored. Water absorbs a very small amount of visible light and you will see only what is not absorbed. I'm not sure what color pure water has. It may appear slightly green.

The sun is a ball of incandescent gas. That gas moves about, flowing up and down as well as across the sun's surface. This movement keeps the sun's temperature roughly uniform but there are occasionally imperfections; regions of the sun's surface that get out of balance with the rest of the sun. When you cook a thick soup on the stove, there will also be regions of the surface that are cooler than others.

Tanning beds emit ultraviolet light in order to trigger your skin's tanning response. This ultraviolet light can and does cause chemical damage to your skin. Like all light, ultraviolet light is absorbed and emitted as particles. The energy in each light particle depends on its wavelength and, since ultraviolet light has short wavelengths, ultraviolet light particles carry lots of energy. They carry enough energy to rearrange the molecules that absorb them. If those molecules are part of the genetic information of a cell, the cell may die or, worse yet, may become cancerous. The shorter the wavelength of the ultraviolet light, the more energetic its particles and the more damage it can do. Tanning beds walk a narrow line between inducing tanning and causing significant damage. Leather skin is one end result of too much chemical damage. Tanning beds that emit relatively long wavelength ultraviolet are probably less harmful than those that emit shorter wavelength ultraviolet (these wavelength ranges are sometimes designated by letters A, B, and C...I think that A is the longest wavelength and least harmful). Still, you skin's tanning response is a defense against chemical damage and is probably not worth trying to trigger with light. Recent research seems to have found chemicals that trigger tanning. These chemicals mimic light-damaged molecules in your skin. Your skin senses these molecules and responds by tanning. If these chemicals work, you'll soon be able to develop a true tan without exposure to light.

The ring that you see surrounding the sun is probably the 22° halo caused by refraction from ice crystals in the upper atmosphere. These tiny ice crystals are hexagonal prisms and they deflect the light that passes through them to form a ring of light around the sun. Because the particles are large enough to bend all the colors of light equally, the ring appears white--or blue-white when superimposed on the blue sky. The deep blue of the surrounding sky is caused by Rayleigh scattering of the sunlight passing through it. In this process, small groups of air molecules and tiny dust particles deflect sunlight toward your eye. Since they deflect short wavelength light (blue light) more effectively than long wavelength light (red light), they give the sky a bluish glow. Finally, the white appearance of the horizon is probably light scattered toward your eyes by surface haze. Relatively large particles in the air scatter sunlight in all directions so that you see a white glow from the air near the ground.

A wonderful reference for some of these ideas is "Rainbows, Halos, and Glories" by Robert Greenler.

The water droplets in clouds are quite large; large enough to be good antennas for all colors of light. As light passes by those droplets, some of it scatters (is absorbed by the antenna/water droplets and is reemitted by the antenna/water droplets). Since there is no color preference in this scattering from large droplets, the scattered light has the same color as the light that illuminated the cloud. In the daytime, the sunlight is white so the clouds appear white. But at sunrise or sunset, the sun's light is mostly red (the blue light has been scattered away by the atmosphere before it reached the clouds) so the clouds appear red, too. If the clouds are very thick, they may absorb enough light (or scatter enough upward into space) to appear gray rather than white. Another way to see why the clouds are white is to realized that light reflects from every surface of the water droplets. As the light works its way through the random maze of droplets, it reflects here and there and eventually finds itself traveling in millions of random directions. When you look at a cloud, you see light coming toward you from countless droplets, traveling in countless different directions. You interpret this type of light, having the sun's spectrum of wavelengths but coming uniformly from a broad swath of space, as being white. These two views of how light travels in a cloud (absorption and reemission from droplets or reflections from droplet surfaces) turn out to be exactly equivalent to one another. They are not different physical phenomena, but rather two different ways to describe the same physical phenomena.

When light reflects from a horizontal surface at an angle, the reflected light tends to be polarized horizontally. At a specific angle, Brewster's angle, the light is completely horizontally polarized because any vertically polarized light that hits the surface at this angle is allowed to enter the surface without reflection. Since reflections from horizontal surfaces are mostly horizontally polarized, glare is mostly horizontally polarized. Polarizing sunglasses deliberately block horizontally polarized light to reduce glare.

Black light is ultraviolet light. You cannot see it so a room illuminated only by ultraviolet light appears dark or "black". However any fluorescent materials in the room (e.g. brighteners in your clothes) will absorb the ultraviolet light and reemit it as visible light. That is why things with fluorescent pigments on them glow when illuminate by black light.

The sun appears bluer when viewed from outside our atmosphere. The earth's atmosphere scatters a substantial fraction of the violet and ultraviolet light in sunlight, leaving a reddened sun disk to our view. Without that scattering, the sun's disk will appear to contain more blue and ultraviolet light.

These colors come from the atomic fluorescence of particles high above the earth's surface. As charged particles from the sun's "solar wind" spiral through the earth's magnetic field toward its poles, they collide with one another and with atoms in the earth's upper atmosphere. The energy of such collisions can excite the atoms involved and cause them to emit light.

The magnifying glass is a lens, a carefully shaped piece of glass that can refract sunlight to create an image. When you burn wood with a magnifying glass, you are creating an image of the sun on the wood. This tiny image, a circle that looks just like the sun itself, only much smaller, is so bright and contains so much thermal radiation that it overheats the wood that it strikes and causes that wood to burn.

Yes, refraction is used in eye glasses. By carefully sculpting the front and back surfaces of a sheet of glass or plastic, the light passing through that sheet can be bent in remarkable ways. We will look at image formation in the section on Cameras.

Just because two materials both reflect all of the light that strikes them doesn't mean that they look the same. When you send a flashlight beam at a white surface, you can see that reflected light from all directions. When you send the flashlight beam at a mirror surface, you can only see the reflected light from one particular angle. Both the white surface and the mirror surface reflect virtually all of the light that hits them. A shiny white surface is different from a dull white surface because a shiny white surface has a small amount of mirror character to it: you can see the whiteness from any direction but there is also a mirror aspect that you can only see from certain angles.

Suntan lotion (or rather sunscreen) is a chemical whose molecules absorb ultraviolet light and turn its energy into heat. Like fluorescent compounds, these molecules absorb ultraviolet light strongly. But unlike fluorescent compounds, the sunscreen molecules do not reemit any light. They convert all of the ultraviolet light energy into heat, which does no damage to your skin.

Actually, some light is heat. Heat is the energy that flows from one object to another because of a difference in their temperatures. The sun is hotter than you are so that it sends heat toward you. Sunlight is heat; it is the sun's heat being sent toward you as electromagnetic radiation. When it strikes the surface of your skin, this radiation is absorbed and becomes the more familiar form of heat: kinetic and potential energy in the atoms and molecules. From the surface of your skin, this heat flows inward to warm the rest of your body. Any material that absorbs light usually converts it to heat. The charged particles in that material move under the influence of the light's electric field and these moving charged particles transfer their energy here and there as heat.

When two identical waves (usually two halves of the same wave) arrive together out of phase, the electric field in one wave (or half-wave) is up at the same moment that the electric field of the other wave (or half-wave) is down. These two electric fields add together and create a total electric field that is neither up nor down. An electric charge at this location in space will experience no forces so there is no electric field (one wave pushes that charge up while the other wave pushes that charge down). With no electric field around, there is no light to be absorbed. If two waves coming toward you interfere destructively, you will see no light. You might worry about conservation of energy; where did the light and its energy go? It went somewhere else. Any time there is destructive interference at one point in space, there will always be some other point in space at which there is constructive interference. Thus when you look at a soap film and see no red light, you can be sure that the red light has gone somewhere else. In the case of the soap film, when you see no red light in the reflection from the film, that red light has been transmitted by the film and is visible on the opposite side of the film.

Some of them may be polarized materials, blocking horizontally polarized light, but most are simply absorbing materials that are embedded directly in the glass during its manufacture. Chemically tinted glass just darkens the sky be absorbing some of the light passing through the glass, regardless of polarization. It's not possible to chemically treat the glass to make it absorb only one polarization of light because that treatment would have to carefully align its molecules. In the plastic polarizing sheets, there is an alignment process (usually stretching in one direction) that lines up all the absorbing molecules.

Light from the sun travels in straight lines (apart from some wave effects called diffraction, that are unimportant in this case). As sunlight passes objects, those objects absorb or scatter the sunlight, leaving regions of space that no longer contain any electromagnetic waves. Regions of space behind the objects contain no sunlight and do not appear illuminated. We perceive those dark, unilluminated regions as shadows.

The sheet polarizers that are used in sunglasses or in the demonstrations in class contain molecules that absorb electromagnetic waves of only one polarization. These molecules form long chains that interact with electromagnetic waves only when the electric fields push charge along the lengths of the molecules. In the polarizing sheets, the molecules are all oriented along the same direction so that they all absorb light of the same polarization. The other polarization of light passes through the sheets virtually unscathed. When unpolarized (randomly polarized) light enters one of these sheets, any waves that are polarized along the molecules are absorbed while any that are polarized across the molecules are permitted to pass. About half the light makes it through and that half is polarized across the molecules. If this remaining light is sent through a second polarizing sheet, turned 90° so that the molecules of the second sheet are aligned with the polarization of the light leaving the first sheet, then the remaining light will be absorbed in the second sheet and essentially no light will emerge from the pair of sheets. This arrangement, two polarizers turn 90° with respect to one another, is called "crossed polarizers". It is a useful arrangement for observing materials that rotate polarization by distorting the electric and magnetic fields. If a distorting material is placed between the two crossed polarizers, light from the first polarizer may be altered by the material and thus be able to pass through the second polarizer.

A thin layer of oil on water creates interference effects, just like those seen in a thin soap film. Sunlight reflects from both the top and the bottom of the oil layer and these two reflections can interfere with one another. If the blue/green wavelengths of light interfere destructively on their way to your eye, you will see the oil layer as red. If the green/red wavelengths of light interfere destructively, you will see the oil layer as blue. How you see the oil layer depends on its thickness and the angles of the light.

The people who invented that tale were well aware of the impossibility of reaching the rainbow itself. Knowing that the rainbow moves with you, they were free to promise anything about what lies at the base of the rainbow.

Yes, it forms an arc that extends to the ground. However, any hills or valleys may obscure its visibility or its sunlight, so you often see it truncated or in shadow.

Blue light almost always bends more than red light because blue light almost always travels more slowly through glass than does red light. This phenomenon is known as dispersion However, there are some glasses that exhibit anomalous dispersion, where red light travels faster and bends more than blue light. Anomalous dispersion only occurs when there is a resonant absorption of light in the glass, typically because of some impurity atoms or ions in the glass or because of some transition that occurs in the glass itself. While the resonance will only absorb light at one particular wavelength, it alters the propagation of light at nearby wavelengths. At wavelengths just shorter than the absorbed wavelength, light travels anomalously fast through the glass so that it bends less than light that is somewhat redder in color.

Yes. Pollution does tend to make the sky bluer and the sunsets redder. However, pollution also imparts colors directly by absorbing certain wavelengths of light. The orange haze that hovers over cities is often caused by nitrogen oxides, which are simply orange in color and act like pigments to make everything appear orangish. However smoke and dust certainly change the look of the sky by increasing scattering. Natural disasters are even more effective: volcanic eruptions create the most beautiful sunsets of all by tossing vast amounts of dust into the air.

Not exactly. A puddle contains water, which reflects light directly. Light from the blue sky travels toward the puddle and illuminates it. As the light enters the water, with its higher refractive index, part of the light reflects. You see this light when you look at the surface of a puddle. But a mirage involves refraction (bending) of light. As light from the blue sky enters a regions of hot air near the surface, that light bends upward. You again see light from the sky, but bent upward by the air rather than being reflected upward by a surface of water. Since the two appear similar, you interpret the shimmering blue light of a mirage as coming from a pool of water. But it is just hot air.

No. Such sunglasses would absorb all light and would appear black. Polarizing sunglasses are designed to absorb only horizontally polarized light; the light associated with glare. There is no reason to absorb vertically polarized light.

If the sky were uniformly filled with water droplets and uniformly illuminated with sunlight, then you would always see the rainbow, no matter where you moved. However it would always appear out in the distance. The light that reaches your eyes as the rainbow comes from a broad range of distances, but it appears to come from pretty far away. As you walked toward this perceived rainbow, you would begin to see light from other raindrops, still farther away. You could never actually "reach" the rainbow. It would just move about with you; always appearing to be in the distance.