Atmospheric Optical Phenomena

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 make 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.