Atmospheric optical phenomena
Atmospheric optical phenomena
Atmospheric optical phenomena are visual events that take place in Earth’s atmosphere as a consequence of light reflection, refraction, and diffraction by solid particles, liquids droplets, and other materials present in the atmosphere. Such phenomena include a wide variety of events ranging from the blue color of the sky itself to mirages and rainbows to sundogs and solar pillars.
The fact that colors appear in the atmosphere is a consequence of the way that white light is broken up into its component parts—red, orange, yellow, green, blue, indigo, and violet (the spectrum)—during its interaction with materials in the atmosphere. That interaction takes one of three general forms: reflection, refraction, and diffraction.
Reflection occurs when light rays strike a smooth surface and return at an angle equal to that of the incoming rays. Reflection can explain the origin of color in some cases, because certain portions of white light are more easily absorbed or reflected than are others. For example, an object that appears to have a green color does so because that object absorbs all wavelengths of white light except that of green, which is reflected.
One form of reflection—internal reflection—is often involved in the explanation of optical phenomena. During internal reflection, light enters one surface
of a transparent material (such as a water droplet), is reflected off the inside surface of the material, and is then reflected a second time out of the material. The color of a rainbow can partially be explained in terms of internal reflection.
Refraction is the bending of light as it passes at an angle from one transparent material into a second transparent material. The process of refraction accounts for the fact that objects under water appear to have a different size and location than they have in air. Light waves passing through water and then through air are bent, causing the eye to create a visual image of the object.
Perhaps the most common example of an atmospheric effect created by refraction is the displacement of astronomical bodies. When the sun is directly overhead, the light rays it emits pass straight through Earth’s atmosphere. No refraction occurs, and no change in the sun’s apparent position takes place.
As the sun approaches the horizon, that situation changes. Light from the sun enters Earth’s atmosphere at an angle and is refracted. The eye sees the path of the light as it is bent and assumes that it has come from a position in the sky somewhat higher than it really is. That is, the sun’s apparent location is displaced by some angle from its true location. The same situation is true for any astronomical object. The closer a star is to the horizon, for example, the more its apparent position is displaced from its true position.
One of the most dramatic examples of sunlight refraction is the green flash. That term refers to the fact that in the moment following sunset or sunrise,
a flash of green light lasting no more than a second can sometimes be seen on the horizon on the upper part of the sun. The green light is the very last remnant of sunlight refracted by Earth’s atmosphere, still observable after all red, orange, and yellow rays have disappeared. The green light remains at this moment because the light rays of shorter wavelength—blue and violet—are scattered by the atmosphere. The green flash is rarely seen.
Light that bounces off small objects is not reflected uniformly, but is scattered in all directions. The process of scattering is responsible for the fact that humans observe the sky as blue. When white light from the sun collides with molecules of oxygen and nitrogen, it is scattered selectively. That is, light with shorter wavelengths—blue, green, indigo, and violet—is scattered more strongly than is light with longer wavelengths—red, orange, and yellow. No matter where a person stands on Earth’s surface, she or he is more likely to see the bluish light scattered by air molecules than the light of other hues.
Stars twinkle; planets do not. This general, though not inviolable, rule can be explained in terms of refraction. Stars are so far away that their light reaches Earth’s atmosphere as a single point of light. As that very narrow beam of light passes through Earth’s atmosphere, it is refracted and scattered by molecules and larger particles of matter. Sometimes the light travels straight toward an observer and sometimes its path is deflected. To the observer, the star’s light appears to alternate many times per second, which produces twinkling.
Planets usually do not twinkle because they are closer to Earth. The light that reaches Earth from them consists of wider beams rather than narrow rays. The refraction or scattering of only one or two light rays out of the whole beam does not make the light seem to disappear. At any one moment, enough light rays reach Earth’s surface from a planet to give a sense of one continuous beam of light.
One of the most familiar optical phenomena produced by refraction is a mirage. One type of mirage— the inferior mirage—is caused when a layer of air close to the ground is heated more strongly than is the air immediately above it. When that happens, light rays pass through two transparent media—the hot, less dense air and the cooler, more dense air—and are refracted. As a result of the refraction, the blue sky appears to be present on Earth’s surface; it may look like a body of water and objects such as trees appear to be reflected in that water.
A second type of mirage—the superior mirage— forms when a layer of air next to the ground is much cooler than the air above it. In this situation, light rays from an object are refracted in such a way that an object appears to be suspended in air above its true position. This phenomenon is sometimes referred to as looming.
The most remarkable phenomenon in the atmosphere may be the rainbow. To understand how a rainbow is created, imagine a single beam of white light entering a spherical droplet of water. As the light passes from air into water, it is refracted (bent). However, each color present in the white light is refracted by a different amount—the blues and violets more than the reds and yellows. The light is said to be dispersed, or separated, according to color. After the dispersed rays pass into the water droplet, they reflect off the rear inner surface of the droplet and exit into the air once more. As the light rays pass out of the water into the air, they are refracted a second time. As a result of this second refraction, the separation of blues and violets from reds and yellows is made more distinct.
An observer on Earth’s surface can see the net result of this sequence of events repeated over and over again by billions of individual water droplets. The rainbow that is produced consists simply of the white light of the sun separated into its component parts by each separate water droplet.
The passage of sunlight through cirrus clouds can produce any one of the optical phenomena known as haloes, sundogs, or sun pillars. One explanation for phenomena of this kind is that cirrus clouds consist of tiny ice crystals that refract light through very specific angles, namely 22° and 46°. When sunlight shines through a cirrus cloud, each tiny ice crystal acts like a glass prism, refracting light at an angle of 22° (more commonly) or 46° (less commonly).
A halo is one example of this phenomenon. Sunlight shining through a cirrus cloud is refracted in such a way that a circle of light—the halo—forms around the sun. The halo may occur at 22° or 46°.
Sundogs are formed by a similar process and occur during sunrise or sunset. When relatively large (about 30 microns) crystals of ice orient themselves horizontally in a cirrus cloud, the refraction pattern they form is not a circle (a halo), but a reflected image of the sun. This reflected image is located at a distance of 22° from the actual sun, often at or just above the horizon. Sundogs are also known as mock suns or parhelia.
Sun pillars are, as their name suggests, narrow columns of light that seem to grow out of the top or (less commonly) from the bottom of the sun. This phenomena is a result not of refraction, but of reflection. Sunlight reflects off the bottom of flat ice crystals as they settle slowly toward Earth’s surface. The exact shape and orientation of the sun pillar depends on the position of the sun above the horizon and the exact orientation of the ice crystals to the ground.
In addition to reflection and refraction, the path of a light ray can be altered by yet a third mechanism, diffraction. Diffraction occurs when a light ray passes close to some object. For comparison, you can think of the way in which water waves are bent as they travel around a rock. Diffraction may also result in the separation of white light into its colored components.
When light rays from the Moon pass through a thin cloud, they may be diffracted. Interference of the various components of white light that generates the colors makes up the corona. The pattern formed by the diffraction rays is a ring around the Moon. The ring may be fairly sharp and crisp, or it can be diffuse and hazy. The ring is known as a corona. Coronas may also form around the sun, although because the sun is much brighter, they are more difficult to observe.
A glory is similar to a corona but is most commonly observed during an airplane ride. As sunlight passes over the airplane, it may fall on water droplets in a cloud below. The light that is diffracted then forms a series of colored rings—the glory—around the airplane’s shadow.
See also Spectrum.
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David E. Newton