Atmospheric optics is the study of how light interacts with the atmosphere and objects in it. It explains, for example, why a mirage occurs, how a rainbow forms, why sunsets are red, and why the sky is blue.

Mirages
A mirage occurs when an object appears displaced from its true position. Atmospheric mirages are created when light is bent, or refracted, as it travels through layers of air with differing densities. Changes in air density are usually caused by changes in air temperature. If the air near the ground is much warmer than the air above, light from the sky will bend up into an observer's eyes so that an observer looking down at the distant ground sees light from the sky. The image of sky where the distant ground should be produces the mirage of a watery pavement, or water resting on hot desert sand. When the light from an object is bent, making the object appear higher than it actually is, a superior mirage occurs. When an object appears lower than it actually is, the mirage is called an inferior mirage.

Rainbows
A rainbow is an arc of concentric colored bands that spans a section of the sky. For a rainbow to form, rain must be falling in one part of the sky and the sun must be shining from behind the observer. Rainbows form when sunlight enters a raindrop and the various wavelengths of visible light, representing the different colors, begin to slow and bend. Violet light bends the most and red light bends the least. Most of the light passes through the raindrop. But the refracted light that hits the back of the drop at a certain angle (called the critical angle) is reflected off the back of the drop. The light is then refracted, or bent, a second time as it emerges from the drop. Because each color bends differently, each color emerges from the drop at a slightly different angle, producing a spectrum of colors. Because only a single color from each drop reaches an observer, it takes many raindrops, each one reflecting light back to an observer at slightly different angles, to produce the colors of a primary rainbow.


Primary Rainbow
A rainbow forms when raindrops separate white sunlight into a spectrum. As sunlight passes through a drop of water, it is first bent and then reflected from the back surface of the drop toward the viewer's eye. The amount of bending, known as refraction, differs for light of different colors. Red light bends the least and violet light bends the most. Here, a primary rainbow arcs through the sky after a rainstorm. Primary rainbows have red on the outside arc; the colors of secondary bows are reversed .

Fainter, secondary rainbows often form above the primary rainbow. Secondary rainbows form when sunlight enters a raindrop at such an angle that two reflections occur inside the raindrop. The second reflection weakens the light intensity and causes a reversal of colors. The weakened light that emerges produces a dimmer rainbow.