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.
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