During sunrise and sunset, the disc of the sun actually contains all the colors of the spectrum, although green and blue light are more scattered, causing the disc to look more red to orange. Under certain atmospheric conditions, however, a brief flash of green, or more rarely blue, will appear. This is known as a “green flash,” and it is not an optical illusion, although it might feel like one. A green flash usually only lasts for an instant, and it is often missed by casual observers.
Originally, scientists theorized that a green flash was an optical illusion caused by looking directly at the sun when it was near the horizon. However, photography and more extensive studies of the phenomenon have proved that this theory is incorrect. A green flash forms because of the unique properties which light has. As the sun sets, the light it emits is scattered by the atmosphere. When the sun nears the edge of the horizon, it begins to be distorted in both shape and color. The result is a green flash, when the conditions are favorable for it.
The sun appears slightly flattened as it sets, due to distortion by the atmosphere. With special lenses, it is possible to see a faint yellow disc slightly above the red setting sun, followed by green, blue, and violet. These bands of color are created by differences in the density and temperature of the Earth's atmosphere. If the weather is extremely clear, as the sun slips below the horizon, it will appear to flash quickly through all the colors of the spectrum. The result is a a green or blue flash, a momentary appearance of a bright green dot or bar on the horizon. Sometimes, a green flash takes the form of a ray which appears to be projecting from the rapidly setting sun.
Conditions are not always right to see a green flash, although it technically happens every time the sun rises and sets. Because of the unusualness of green flashes, they have come be associated with mystical events. It also sometimes creeps up in popular culture, with green flashes playing a role in films like Pirates of the Caribbean and in some books as well.
Be careful when looking for a green flash. The setting sun can still cause eye damage, especially if it is stared at for long periods of time. Since green flashes normally appear once the disc of the sun has slipped over the horizon, you can quickly glance to gauge the sun's position, only focusing once the time for a green flash nears. The sight will last only a moment, so if you have a camera, try to use a sportshot feature to capture the sunset, to ensure that you do not miss the green flash.