Optical Effects

The interaction between sunlight and the atmosphere produces a wide array of patterns and colors of light in the sky, and even optical illusions on the ground. The amount and types of particles (including water and ice) in the air, as well as the position of the Sun in the sky, influence the quality of the light perceived by our eyes.

Sunlight can appear as white light, as a single color of light, or as the entire spectrum of colors of light. The Sun itself may appear as a ringed image or as multiple images. Sunlight can also be distorted to create mirages, such as apparently wet roadways or towering mountains on the horizon.

These and other atmospheric optical phenomena are produced under specific and often unique conditions. Most of these phenomena take place during the day although some, caused by the interaction of moonlight and the atmosphere, occur at night. The visual displays that take place in the outermost reaches of Earth's atmosphere, the aurora borealis and aurora australis (also called the northern and southern lights), are caused by charged particles from the Sun rather than sunlight itself.

The color of light

About 45 percent of the solar radiation that reaches Earth's atmosphere is in the form of visible light. The electromagnetic spectrum includes the entire array of electromagnetic radiation, and visible light is that portion of the electromagnetic spectrum that we can see. Visible light includes the wavelengths of every color, from that with the longest wavelength, red, to that with the shortest wavelength, violet. The order of wavelengths of colors, from longest to shortest, can be remembered by using the mnemonic ROYG. BIV: R=red, O=orange, Y=yellow, G=green, B=blue, I=indigo, and V=violet. It is important to remember that violet, as used by physicists and astronomers, is not the same color as a violet crayon or the color of the flowers called violets. It is instead an extremely dark blue color, almost black in appearance.

WORDS TO KNOW

aurora:

a bright, colorful display of light in the night sky, produced when charged particles from the Sun enter Earth's atmosphere.

cirriform:

a wispy, feathery fair-weather cloud formation that exists at high levels of the troposphere.

conduction:

the transfer of heat by collisions between moving molecules or atoms.

corona:

a circle of light centered on the Moon or Sun that is usually bounded by a colorful ring or set of rings.

crepuscular rays:

bright beams of light that radiate from the Sun and cross the sky.

critical angle:

the angle at which sunlight must strike the back of the raindrop in order to be reflected back to the front of the drop.

diffraction:

the slight bending of sunlight or moonlight around water droplets or other tiny particles.

dispersion:

the selective refraction, or bending, of light that results in the separation of light into the spectrum of colors.

electromagnetic spectrum:

the array of electromagnetic radiation, which includes radio waves, infrared radiation, visible light, ultraviolet radiation, X rays, and gamma rays.

Fata Morgana:

a special type of superior mirage that takes the form of spectacular castles, buildings, or cliffs rising above cold land or water.

frontal system:

a weather pattern that accompanies an advancing front.

glory:

a set of colored rings that appears on the top surface of a cloud, directly beneath the observer. A glory is formed by the interaction of sunlight with tiny cloud droplets and is most often viewed from an airplane.

green flash:

a very brief flash of green light that appears near the top edge of a rising or setting Sun.

halo:

a thin ring of light that appears around the Sun or Moon, caused by the refraction of light by ice crystals.

inferior mirage:

a mirage that appears as an inverted, lowered image of a distant object. It typically forms in hot weather.

iridescence:

an irregular patch of colored light on a cloud.

middle latitudes:

the regions of the world that lie between the latitudes of 30° and 60° north and south. Also called temperate regions.

mirage:

an optical illusion in which an object appears in a position that differs from its true position, or a nonexistent object (such as a body of water) appears.

reflection:

the process by which light both strikes a surface, and bounces off that surface, at the same angle.

refraction:

the bending of light as it is transmitted between two transparent media of different densities.

scattering:

multidirectional reflection of light by minute particles in the air.

superior mirage:

a cold-weather mirage that appears as a taller and closer, and sometimes inverted, image of a distant object.

troposphere:

the lowest atmospheric layer, where clouds exist and virtually all weather occurs.

Visible light is separated into its spectrum of colors when it passes through a glass prism or another medium, such as an ice crystal or raindrop. The prism bends each component of white light to a different degree, depending on that component's wavelength. For instance, red light, which has the longest wavelength, is bent the least. Violet light, which has the shortest wavelength, is bent the most. As a result, the entire rainbow of colors exits the prism, with red and violet on opposite ends.

Sunlight is white because it contains all visible wavelengths of light. The color of objects is caused by the fact that they absorb some wavelengths of light and reflect others. Reflection means that light bounces off a surface at the same angle that it strikes a surface. This definition will become important later in this chapter, when we compare reflection to refraction, which is the bending of light.

A white shirt, for example, does not absorb any wavelengths of visible light. The shirt reflects all wavelengths of visible light, which causes it to appear white. On the other hand, the skin of a red apple absorbs all wavelengths of radiation except red. Red light is reflected by the apple skin, which is the reason it appears red. An object that absorbs all wavelengths and reflects none appears black.

The scattering of light

When sunlight encounters minute particles in the atmosphere, such as air or water molecules, or small particles of dust, it reflects off them in every direction. Sunlight is, in effect, bounced around like a pinball by these particles. This multidirectional reflection is called scattering.

Blue skies

The scattering of sunlight by air molecules is what causes the sky to appear blue. However, it is a selective scattering, meaning that not all wavelengths of visible light are scattered equally. Air molecules scatter primarily violet, indigo, blue, and green light, the colors at the short-wavelength end of the visible spectrum.

The small size of air molecules is responsible for the selective scattering of sunlight. The diameter of an air molecule is even smaller than the average wavelength of visible light. Air molecules are therefore better able to scatter shorter wavelengths of visible light than longer wavelengths.

When you look at the sky, your eye is bombarded from all directions by violet, indigo, and blue light. However, the structure of the human eye is such that the eye is much more sensitive to blue light than the other colors. Thus, when violet, indigo, and blue light are present at once, what the eye perceives is blue.

If there were no air molecules or other particles in the air, and therefore no scattering, the sky would appear black.

The scattering of light by clouds

A cloud droplet has a far greater diameter than that of an individual air molecule. A cloud droplet, then, is capable of scattering all wavelengths of visible light to a fairly equal extent. In addition, a cloud droplet is a poor absorber of light. As described above, an object that reflects the entire spectrum of visible light appears white.

The amount of sunlight that penetrates a cloud depends on the thickness of the cloud. Some of the sunlight that strikes a small cloud will pass through the cloud, while the rest is reflected by the droplets it encounters. The sunlight that makes it through the cloud is scattered by droplets near the base, which is what makes the base of small clouds appear white.

However, very little sunlight will reach the base of a tall cloud with a thickness of 3,300 feet (1,000 meters) or greater. That is the reason that the base of a cloud with vertical development appears dark. The base of a cloud also darkens as its droplets become larger. The reason for this is that larger droplets of water are better at absorbing light than are smaller units. Therefore, the darkness of the base of a cloud is also an indication of the likelihood of rain.

Haze

Haze is the term used to describe a sky that has a uniform, milky white appearance. Haze is produced by high humidity in combination with a large number of particles in the air. Water vapor condenses around the suspended particles. These "haze particles," as they are called, scatter all wavelengths of visible light in all directions, just as cloud droplets do. The greater number of particles in the air, the whiter the sky appears.

The concentration of particles is often an indicator of air pollution, since the particles come mainly from emissions from smokestacks or automobile tailpipes. Haze may also be created by naturally occurring particles in the air, such as pollen and dust.

Haze occurs close to the surface. If you climb to the top of a tall mountain on a hazy day, you may see haze below and blue skies above.

Crepuscular rays

Crepuscular rays are bright beams of light that appear to radiate from the Sun and cross the sky. They are most often visible at sunset or when the Sun shines through a break in the clouds. The beams are made visible by the scattering of sunlight by dust, water droplets, or haze particles.

A similar effect is created when a bright light shines through a small opening, into a dusty room. This effect can be seen in a darkened movie theater where the intense beam of light leaves the projection booth. The beam is visible because it is being scattered by dust in the air.

Despite their appearance of fanning outward from the Sun, crepuscular rays run parallel to one another. The fan shape is only an illusion, caused by perspective. This illusion is similar to that of a road, railroad track, or any other long straight path that appears to narrow to a single point in the distance.

Colors at sunrise and sunset

Why does the Sun appear red, yellow, or orange when it is on the horizon? The answer has to do with the angle at which sunlight strikes a given location. In the middle of the day sunlight strikes the ground most directly, and at the beginning and end of the day sunlight strikes the ground at the steepest angle. The angle at which sunlight strikes the ground is indicative of the amount of atmosphere through which the sunlight must pass.

At sunrise and sunset, the sunlight must pass through the greatest distance of atmosphere. In fact, sunlight passes through about twelve more miles (nineteen more kilometers) of atmosphere when the Sun is just over the horizon than it does when directly overhead. As the light of the setting or rising Sun travels through all that atmosphere, its shorter wavelengths become scattered by the air molecules it encounters. The only wavelengths to make it all the way to Earth's surface are the longest wavelengths: red, orange, and yellow.

When the air is relatively clean, a setting or rising Sun appears to be orange-yellow. An orange-red Sun, however, indicates that the air contains a high concentration of particles. Particles that have diameters slightly larger than air particles scatter yellow wavelengths, leaving only light with the longest wavelengths—orange and red—to shine through. When the concentration of particles in the air is very high, such as after a volcanic eruption, only red light remains. All other colors are scattered and the Sun appears completely red.

The refraction of light

When light passes from one medium into a second medium, its speed changes. For example, when light travels from a less dense medium, like air, into a more dense medium, like water, the light slows down. If the light enters the denser medium at any angle other than from straight above, it will bend. The bending of light, as it passes through two transparent media (plural form of medium) of different densities is called refraction. The degree to which light bends depends on both the densities of the two substances and the angle at which light enters the second substance. When light is transmitted from a less dense substance to a more dense substance, it bends towards the perpendicular to the boundary between the two.

Positions of the Sun, Moon, and stars

The light from the Sun, Moon, and stars travels from a less dense medium, space, to more dense medium, Earth's atmosphere. Thus, all starlight, sunlight, and moonlight, except that emitted when the Sun, Moon, or star is directly overhead, is refracted. As a result, our perception of the positions of the Sun, Moon, and stars is distorted.

When we look at a star in the sky, we are actually seeing the light from that star which has been bent upon entering our atmosphere. Our eyes cannot perceive that the light has been bent, hence they cannot trace the path of light back to the actual position of the star. Therefore, the star appears to be higher in the sky than it actually is.

When a star is near the horizon, its light must pass through the greatest amount of atmosphere, and undergo the greatest amount of refraction before it reaches an observer on the surface, of any position in the sky. Thus, when a star is near the horizon, the star's image appears farthest from its true position.

Experiment: The refraction of light by water

The best way to understand refraction is to try the following simple experiment: Fill a large glass beaker with water. Add a pinch of paprika or other ground spice. The spice causes a scattering effect, which makes the light beam easier to see. Then turn off the lights and shine a small flashlight into the water, from straight above. You will see the beam of light continue straight through to the bottom of the glass. Now tilt the flashlight so that the light enters the water at an angle. You will notice that the beam of light bends slightly toward the perpendicular.

Refraction also causes an observer to see the rising of the Sun or Moon about two minutes before it actually occurs, and the setting of the Sun or

Moon about two minutes after it actually occurs. This situation is similar to what happens with starlight on the horizon. Because the light of the Sun and Moon must shine through so much atmosphere, it is refracted in such a way that they appear higher in the sky than they really are.

The refraction of light near the horizon is also responsible for the subtle "flattening" of a setting or rising Sun or Moon. Specifically, when the Sun or Moon is directly on the horizon, the light from the lower portion of the object is refracted to a greater degree than the light from the upper portion of the object. This causes the object to appear to be wider at the base than at the top.

Green flashes

A green flash is a very brief and difficult-to-see optical effect that accompanies a rising or setting Sun. The green flash is a flash of green light that appears near the top edge of the Sun. The green flash is due to both refraction and scattering of light in the atmosphere.

A green flash occurs because all wavelengths of light from a setting or rising Sun are not refracted equally. Rather, the shorter wavelengths, purple and blue, are bent to the greatest degree, while the longer wavelengths, red and orange, are bent to the smallest degree. Thus, we would expect that the color of the very tip of the Sun as it first peeks over the horizon, or just sinks below the horizon, would be purple-blue. This would be true except that air molecules and tiny dust particles selectively scatter blue and purple wavelengths. The next longest wavelength, green, is what we see instead, unless there is a high concentration of particles in the air. In that case, green light will be scattered as well.

Experiment: Create your own green flash

To learn how a green flash is created, perform this simple experiment. You will need a prism, a lamp (minus the shade), and a piece of dark paper. Hold the prism so it is between yourself and the light. Turn the prism until the entire spectrum of colors can be seen, with red on the bottom and blue on the top. The prism simulates Earth's atmosphere refracting the sunlight.

Then take the piece of dark paper and move it up between the prism and the bulb until only the thinnest bit of the light bulb is showing above the paper. This simulates the Sun about to go completely below the horizon. Move your head up and down until the thin strip of bulb appears green. This simulates the angle at which the green flash would occur.

One reason that the green flash is so elusive is that in most cases, the green light is too faint to be seen by the human eye. The green flash is brightest, and noted most often, over the ocean where the air is relatively clean. It is also more common at high latitudes, where the Sun rises and sets more slowly. At the end of the long polar winter, the sunrise is so gradual that a green flash may persist for several minutes.

Mirages

Another product of the refraction of light in the atmosphere is the mirage. A mirage is an optical illusion such that an object appears in a position different from its true position. Alternatively, a nonexistent object, such as a body of water, may appear. In some mirages, distant objects appear to be inverted or higher or lower than their true positions.

Mirages are caused by the refraction of light as it passes through layers of air with different densities. The differences in density are created by differences in temperature. The sharper the contrast in temperature between two air layers, the more pronounced the refraction of light.

Perhaps the best-known type of mirage is the appearance of "water" commonly seen on a roadway or in the desert. This type of mirage forms on hot days, when the pavement or sand becomes very hot. Heat is transferred from the surface to the air immediately above it by conduction (the transfer of heat by collisions between moving molecules or atoms). In contrast, the air just a few yards above the surface is cooler and denser.

A mirage of water on a road or desert surface is formed by the refraction of blue light from the sky. Rays of blue light from the sky near the horizon travel toward the surface. Near the surface, they encounter a thin layer of superheated air just above the ground. The rays bend upward away from the surface as they are refracted by the less dense hot air. Our eyes thus perceive the blue sky light as coming from just above the surface, in the distance. As we move toward the location where we saw the

mirage, the mirage disappears because the angle of light from the sky becomes too steep. The mirage reappears ahead, moving with us and always remaining in the distance.

Water mirages are even more convincing when they appear to "shimmer," as does water when struck by sunlight. The shimmering of a water mirage is caused by small shifts in the degree to which light is refracted. The reason why the angle of refraction changes is that near the surface hot air is constantly rising and cool air is constantly sinking. This variation causes a continual change in density of air layers which, in turn, causes the continual shift in the amount by which light is refracted.

Inferior mirages

On a hot day, when you look at an object in the distance, you may see an upside-down version of the object directly beneath it. Yet, when you get right up to the object, the inverted image disappears. The upside-down image is an optical illusion called an inferior mirage. Inferior mirages are similar to water mirages. They form when the surface air is hotter and less dense than air at higher elevations.

Light is reflected outward from a distant object, like a tree, in all directions. Some of the light from the treetop travels a straight horizontal path, never dipping into the warmer surface air. When that light reaches your eye, you get an accurate image of the tree. The light from the lower portions of the tree is not refracted because it remains within a medium of a single density, between the tree and your eye.

However, some of the light from the treetop travels at a gentle slope downward and eventually crosses the boundary between air layers. When it does so, it bends upward. When the refracted light reaches your eye, your eye follows the path of the light, at its refracted angle, back over the distance to the tree. Thus, the image appears lower to the ground than it actually is.

The light from the top of the tree is bent upward to the greatest degree, making it appear to come from the lowest position. For this reason, the lowered image of the tree is also upside-down.

Superior mirages

Superior mirages are created under conditions that are opposite of those that create inferior mirages. They form in cold weather when the surface air is colder, and thus denser, than the air above. In a superior mirage, a distant object appears to be taller and closer to the observer than it really is. Sometimes it appears upside down. Superior mirages are most common in polar regions, where the air over a snow-covered surface is colder than the air several feet (about a meter) above.

For example, a mountain in the distance might appear taller and nearer than it actually is. The light from the mountaintop is reflected in all directions. Some of that light follows a gently sloping path downward. When that light enters the layer of colder air, it is bent toward the perpendicular; in this case, into a steeper downward path. When this refracted light reaches your eye, your eye follows the path of the light, at its refracted angle, back over the distance to the mountain. The image thus appears higher above the ground than it normally would.

A special type of superior mirage is called a Fata Morgana. A Fata Morgana takes the form of spectacular castles, buildings, or cliffs rising above cold land or water, particularly in polar regions. This type of mirage is produced by light that is refracted as it passes through air layers of various temperatures. A Fata Morgana requires that the air temperature over a cold surface increase with height. Specifically, the temperature rises slowly throughout the surface layer of air, then several feet above the surface the air temperature rises more quickly. In the next layer of air, the temperature rises slowly again.

Halos

A halo is a thin ring of light that appears around the Sun or the Moon. Halos are caused by the refraction of light by ice crystals. These ice crystals are either free-falling or within upper-level clouds called cirriform clouds. Cirriform clouds are the only type that are both high enough to contain ice crystals and thin enough to allow the image of the Sun to shine through.

There are two main types of halo: the 22° halo and the 46° halo. The 22° halo is smaller, encircles the Sun more tightly, and is more common than the 46° halo. There are several other sizes of halo but they appear very infrequently.

The size of a halo (22° or 46°) refers to the angle by which light is refracted through ice crystals and, consequently, the radius of the halo. For instance, if light is refracted by ice crystals at an angle of 22°, it will form a circle of light with a radius of 22°. To better understand this, draw a picture of a person on the ground and a Moon with a halo, above. Draw two lines: one between the person and the Moon, and the other between the person and a point on the far left or far right side of the halo. The angle formed by the two lines, which in reality would either be 22° or 46°, indicates whether the halo has a radius of 22° or 46°.

Did you know? The origin of Fata Morgana

The name "Fata Morgana" is Italian for "fairy Morgan." According to mythology, Morgan, or Morgan le Fay, was the fairy half-sister of King Arthur. She lived in an underwater crystal palace and was capable of creating magical castles out of thin air. In the fifteenth century, Italian poets from the town of Reggio viewed a fantastic, castle-like mirage near the Strait of Messina (the waterway between Italy and Sicily). Unable to explain what they saw, they called it a "Fata Morgana," and the name stuck.

Both 22° and 46° types of halo are formed when light strikes small, pencil-shaped, hexagonal ice crystals that are around 0.0008 inches (20 micrometers) in diameter. The ice crystals that form a 46° halo may be as small as 0.0006 inches (15 micrometers) or as large as 0.001 inches (25 micrometers), while the ice crystals that form a 22° halo are more uniformly 0.0008 inchs (20 micrometers).

A 22° halo is the result of refraction by randomly oriented ice crystals. The light enters one of the six sides and exits through another of the six sides. In the process, the light is bent by an angle of 22°.

The ice crystals that produce a 46° halo are oriented in such a way that sunlight strikes one of the six sides and exits through one of the two ends. This arrangement causes the sunlight to be refracted at an angle of 46°.

Halos may form at the leading edge of a frontal system, which is the weather pattern that accompanies an advancing front. Thus, they are often looked upon as a sign of rain. A halo is certainly not a foolproof forecasting tool, however, because the front may change direction or gently pass through without producing rain.

Sun dogs

Sun dogs are also called mock suns or perihelia, Greek for "beside the Sun." They consist of one or two patches of light that appear on either or both sides of the Sun. Sun dogs make it appear that there are two or three suns in the sky. When two sun dogs occur, one may be brighter than the other, or higher than the other. They may appear white or colored. Sun dogs often appear just outside the circumference of a 22° halo.

Occasionally these patches of light are seen around a very bright, full moon. In that case, they are called moon dogs.

Sun dogs are produced by the refraction of sunlight that shines through plate-like ice crystals with diameters around 50 micrometers (.0019 inch) or larger. Aerodynamic drag causes the plate-like ice crystals to fall slowly through the air much like leaves falling from a tree. When the ice crystals are positioned horizontally, with large, flat ends parallel to the ground, they will refract sunlight at an angle of 22° and produce the sun dogs. When the ice crystals are randomly oriented, a 22° halo is produced. It takes millions of falling ice crystals, all oriented so that they refract sunlight at 22°, to produce sun dogs.

Where these falling ice crystals are relatively large and plentiful, the sun dogs will be colorful. This color is produced by the selective refraction of light, also called dispersion. In the process of dispersion, each ice crystal acts like a tiny prism, separating sunlight into the spectrum of colors.

The amount by which each color is refracted by an ice crystal varies slightly. Red light has the longest wavelength and is slowed the least as it passes through the ice crystal. Hence, red is bent the least. On the other extreme, violet light has the shortest wavelength and is slowed the most as it passes through the ice crystal. Hence, violet is bent the most.

The result is that red light appears on the edge of the sun dog closest to the Sun and blue appears on the edge farthest from the Sun. The reason why blue, and not violet, appears is that the human eye is better able to perceive blue than violet.

Occasionally a halo will also be colorful, rather than its characteristic white. This dispersion of sunlight into bands of color, by the process just described, occurs when the ice crystals are relatively large and of uniform size and shape.

Rainbows

A rainbow is an arc of light, separated into its different colors, that stretches across the sky. Rainbows are products of both reflection and refraction of sunlight by raindrops. A rainbow is, in effect, sunlight that has undergone dispersion and is reflected back to your eye. To observe a rainbow, the sun must be at your back and the falling rain must be in front of you.

A rainbow is formed by a rather complex process. As sunlight enters a raindrop, it is dispersed into its constituent colors, meaning that each color of the spectrum is refracted to a different degree. Most of this dispersed sunlight passes right through the raindrop. However, when sunlight strikes the back of the raindrop at a certain angle, called the critical angle, the sunlight is reflected back to the front of the drop. To achieve this critical angle, the Sun can be no higher than 42° above the horizon.

As a result of dispersion, once the sunlight enters the raindrop, each color strikes the back of the raindrop at a slightly different angle. Thus, each color reflects off the back of the raindrop and emerges from the front of the raindrop at a slightly different angle.

Only one color exits from each raindrop at the exact angle necessary to reach your eye. This means that you see only one color at a time reflecting from each raindrop. For this reason, it takes millions of raindrops to create a rainbow.

Due to its angle of refraction, red light is reflected to your eye from the highest raindrops. Therefore, red is the color at the top edge of the rainbow. Violet light, which is reflected from the lowest raindrops, forms the bottom edge of the rainbow. The rest of the spectrum—orange, yellow, green, blue, and indigo—fills in the middle portion of the rainbow.

Each time you move, the rainbow you observe is being reflected from a whole different set of raindrops. Each raindrop produces only one ray of light at the appropriate angle to intercept your eye. By the same token, no two people can observe exactly the same rainbow!

Sometimes two rainbows appear in the sky at once. The brighter rainbow, formed by the process just described, is the primary rainbow. The fainter rainbow is called the secondary rainbow. A secondary rainbow is formed when sunlight strikes the raindrops at such an angle that the light is reflected twice within each drop. This double reflection causes violet light to be reflected to the eye from higher raindrops and red light, from lower raindrops. Therefore, in the secondary rainbow, the order of colors is reversed, with violet on top and red on the bottom. Some light is lost in the double-reflection process, which is the reason the secondary rainbow is dimmer than the primary one.

The diffraction of light

Diffraction is the slight bending of sunlight or moonlight around water droplets or other tiny particles that it encounters. The diffraction of sunlight or moonlight produces patches of white and colored light in the sky.

Coronas

A corona (Latin for "crown") is a circle of light centered on the Moon or Sun that is usually bounded by a colorful ring or set of rings. Coronas are difficult to observe around the Sun because of the Sun's brightness. Moonlight coronas are more easily observed.

A corona is the product of the diffraction of sunlight or moonlight around tiny, spherical cloud droplets. A corona can form only when the Sun or Moon is visible through a thin layer of clouds.

The smaller the cloud droplets, the greater the angle of diffraction, and the larger the corona. The largest coronas are produced by a newly formed cloud of uniform thickness. Coronas are much smaller than halos, however, because the angle of diffraction that produces a corona is only a few degrees.

Sometimes alternating light and dark bands are visible in the middle portion of a corona. Light and dark bands are formed when light waves, which have bent around a water droplet, come back together and recombine.

Light waves are similar to water waves in that they also have crests and troughs. When the crest of one wave meets the crest of another wave, the two are added together and become one large wave. This phenomena is called constructive interference. In light waves, constructive interference produces a bright band.

The opposite of constructive interference is destructive interference. Destructive interference occurs when the crest of one wave meets the trough of another and they cancel each other out. When destructive interference occurs in water waves, a calm spot is produced. When it occurs in light waves, a dark band is produced.

The colored rings on the edges of a corona are produced by diffraction of moonlight around cloud droplets of uniform size. If the droplets are different sizes, the color will appear in an irregular, and not a circular, pattern.

In a process similar to that of dispersion, diffraction causes the differential bending of light, according to wavelength. When white light is bent, its longest-wavelength component, red, bends the least and its shortest-wavelength component, violet, bends the most. In this way the light is separated into its constituent colors.

Experiment: Make your own rainbow

You can create your own rainbow using a clear glass bowl of water, a flashlight, and a small, flat mirror. The water acts as a refractor and the mirror acts as a reflector.

Simply place the mirror in the bowl of water, so that it rests against the side of the bowl at about a 45° angle. Then shine a flashlight straight down at the mirror. A rainbow will appear on the wall opposite the mirror.

Red appears on the side of the ring farthest from the Moon and violet appears on the side of the ring closest to the Moon. The corona may have several rings, which become fainter with distance from the Moon.

Iridescence

Iridescence is the term used to describe irregular patches of colored light on clouds. This effect is most often seen within 20° of the Sun or Moon. Iridescence often appears as pastel shades of blue, pink, or green. The brightness of the colors is proportional to the number of droplets within the cloud and the uniformity of size of those droplets.

Iridescence forms in the same way as a corona and is, essentially, an irregular corona. One difference between the two phenomena has to do with the size of the cloud droplets. Iridescence is formed when sunlight is diffracted by cloud droplets of different sizes, while coronas require cloud droplets of a uniform size.

Sometimes iridescence appears as an arc, or a portion of a corona. This is the case when a cloud partially obscures the Sun or Moon, but does not cover the entire region in which a corona would form. Iridescence may also form on a cloud that is near, but not covering, the Sun or Moon.

Glories

A glory is a set of colored rings that appears on the top surface of a cloud, directly opposite the Sun from an observer. Although it is possible to view a glory by climbing a mountain until you're above the clouds, it is much easier to view one from an airplane window. Because they are most often viewed from airplanes, glories are generally thought of as the rings of color that surround the shadow of an airplane.

A glory is formed by a complex process similar to the formation of a rainbow. The main difference between the two phenomena, however, is that rainbows are formed by the interaction of sunlight with raindrops while glories are formed by the interaction of sunlight with tiny cloud droplets. The cloud droplets are less than 50 micrometers (about 0.002 inch) in diameter. In contrast, the droplets that form rainbows are around 0.04 to 0.24 inches (0.1 to 0.6 centimeters) in diameter.

In a glory, sunlight undergoes refraction, reflection, and diffraction within cloud droplets before being returned to your eye. First, sunlight that strikes the surface of a cloud droplet is refracted within the droplet. This refracted light then reflects off the back of the droplet. Some of this reflected light skims the opposite surface of the droplet and bends slightly, or is diffracted, around the droplet. The light then exits the droplet on a path that is parallel to its entry path.

The process of diffraction is also what separates the light of a glory into its constituent colors. As the light is diffracted by cloud droplets, red is bent the least and violet, the most. Hence, as with a rainbow, each droplet reflects light of only one color. The innermost ring of the glory appears purple and the outermost ring appears red, with the rest of the spectrum lying in between.

The Brocken Spectre

A Brocken Spectre (specter or ghost) is the apparently huge and distorted shadow cast by an observer on the tops of clouds that are below the mountain on which the observer stands. The name comes from the Brocken, the tallest peak of the Harz mountain range in Germany. The peak of the Brocken is often above the cloud level and the area is usually misty, so the conditions are often favorable to see a shadow cast onto a cloud layer. These shadows seem to move by themselves because the shape of the cloud layer constantly changes.

The same conditions that can produce a specter are also favorable for the observation of a glory. The appearance of giant shadows that seemed to move by themselves and which were surrounded by optical glories are possibly what gave the Harz mountains the reputation as a refuge for witches and evil spirits. In Johann Wolfgana Goethe's Faust, a famous play about a man who makes a bargain with the devil, the Brocken is called the Blocksberg and is the site of the Witches' Sabbath on Walpurgis Night, which is the night before May Day (May 1).

A glory is always positioned directly beneath the observer or opposite from the Sun. Like a rainbow, a glory moves with the observer. Thus, if you were on one airplane and another airplane was flying beside you, you would be able see the glory around your own plane's shadow, but not the glory around the other plane's shadow.

Auroras

Unlike the other optical phenomena described in this chapter, auroras are not produced by sunlight or moonlight but by radiation from the Sun. Auroras are bright, colorful displays of light in the night sky. They come in two forms, aurora borealis and aurora australis, better known as the northern and southern lights. Auroras are most prominent near the North and South poles, but can be seen occasionally in the regions of the world that lie between the latitudes of 30° and 60° north and south, called the middle latitudes.

A display of northern or southern lights can be as fascinating as fireworks. They vary in color from whitish-green to deep red and take on shapes such as streamers, arcs, curtains, and shells.

Auroras are produced when charged particles from the Sun enter Earth's atmosphere. As this stream of particles approaches Earth, it is trapped for a time in the outermost parts of the Earth's magnetic field. Eventually the particles are drawn down toward the north and south magnetic poles. Along the way, they ionize (create an electric charge within) oxygen and nitrogen gas in the atmosphere. This causes the atmosphere to glow.

[See AlsoClimate; Clouds; Human Influences on Weather and Climate; Weather: An Introduction ]

For More Information

BOOKS

Adam, John A. Mathematics in Nature. Princeton: Princeton University Press, 2003.

Greenler, Robert. Rainbows, Halos, and Glories. Milwaukee, WI: Elton-Wolf Publishing, 2000.

Lee, Raymond L., Jr., and Alistair B. Fraser. The Rainbow Bridge: Rainbows in Art, Myth and Science. Pennsylvania State University Press, 2001.

Lynch, David K., and William Livingston. Color and Light in Nature. 2nd ed. Cambridge, UK: Cambridge University Press, 2001.

PERIODICALS

Laven, Phillip. "How Are Glories Formed?" Applied Optics. (September 2005): pp. 5675-5683.

WEB SITES

Allgeyer, Robert. "APPENDIX: The Fata Morgana Mirage over Monterey Bay." View to the Horizon. 〈http://www.icogitate.com/∼ergosum/essays/vtth/viewtothehorizon.htm〉 (accessed March 23, 2007).

Cowley, Les. "Rainbows." Atmosphere Optics. 〈http://www.atoptics.co.uk/rayshad.htm〉 (accessed March 23, 2007).