A green flash, which occurs more commonly at sunset — but can also occur at sunrise — is a phenomenon in which part of the sun can be observed suddenly and briefly changing color. It usually lasts only a second or two — which is why it is referred a flash — as the sun changes from red or orange at sunset, for example.
The green flash is viewable because refraction bends the light of the sun. The atmosphere acts as a weak prism, which separates light into various colors. When the sun's disk is fully visible above the horizon, the different colors of light rays overlap to an extent where each individual color can't be seen by the naked eye.
When the sun starts to dip below the horizon the colors of the spectrum disappear one at a time, starting with those with the longest wavelengths to those with the shortest. At sunrise, the process is reversed, and a green flash may occur as the top of the sun peeks above the horizon.
It is a primarily a green flash because more green light gets through and therefore is more clearly seen. Sometimes, when the air is especially clear, enough of the blue or violet light rays make it through the atmosphere, causing a blue flash to be visible. However, green is the most common hue reported and captured in photos.
Most green flashes fall into two categories: inferior mirage flashes and mock mirage flashes. Inferior mirage flashes, which accounts for about two-thirds of all green flash sightings, are oval and flat and occur close to sea level and when the surface is warmer than the air above.
Mock mirage flashes occur higher up in the sky and when conditions on the surface are colder than the air above. The flashes appear to be thin, pointy strips being sliced from the sun. [Image Album: Strange & Shining: Gallery of Mysterious Night Lights]
Where to see a green flash?
While there is no optimal condition that will guarantee a green flash sighting, a green flash is best observed with a clear view of the horizon and in an area that is free of pollution. It is more likely to see a green flash when there is visibility of several miles, almost to the point of the curvature of the earth, and the sky is cloudless.
Green flash sightings frequently occur at the ocean, where more of the atmosphere is visible and the line of sight is virtually parallel to the horizon. Prairies also have the appropriate conditions for a green flash sighting.
One popular but inaccurate explanation attributes green flashes to the sun shining through the waves on the ocean. However, the light entering a wave is bent downward, into the water and cannot escape.
Another common misconception is that a green flash is afterimage, a result of the saturation of the red cones in the human retina or that it was an enduring image that remained after the sun had set. But sightings of the green flash at sunrise made both of these ideas implausible.
There have been some mentions of green flashes in popular culture. The heroine of Jules Verne's 1882 novel "The Green Ray" searches for the phenomenon. Polar explorer Admiral Richard Byrd and his crew claimed to have seen a "green sun" persist for 35 minutes while on an expedition to Antarctica. Scientists now believe that there must have been some mirage effect present for the phenomenon to go on for that length of time.