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What is the famous "green flash" all about at sunset?
Question Date: 2006-10-24
Answer 1:

The famous "green flash" at sunset is a scientific phenomenon that has to do with the refraction and dispersion of light. It is rarely seen because it requires specific conditions but it is scientifically possible. It may also be possible to see similar flashes during sunrise as well.
To explain the most common cause of the green flash we need to think about the dispersion of light. White light is made up of light in the colors of the rainbow. These colors have different energies and so pass through matter at slightly different speeds. Shorter wavelengths of light (those with higher energies -blue light) are more efficiently scattered out of the beam of light as they pass through air particles (why the sky is blue). Dispersion of light also depends on the density of the matter it is passing through. The light is also refracted - it bends towards more dense matter. The atmosphere is more dense closer to the surface of the earth. And so the light bends down towards the surface of the earth. Now, the part more difficult to understand is that when the sun nears the horizon some of the light you see is coming from below the horizon. I like to think of this as a mirror image - a mirage, sort of like what you see when driving on a hot day and the road surface seems to shimmer in heat waves reflecting the sky. Under the right conditions -if the air temperature near the earth's surface is higher than the air above it, the hot air will act like a mirror. As the sun dips below the horizon there will be a few seconds of time when the image of the tip of the sun going down and the reflection is magnified. Then due to the dispersion of light and the angles of refraction, the green color of light can be seen.
The best bet for observing this phenomenon? Observe the sunset from lower altitudes (sea level)on a very clear day where the sun is setting on a very flat horizon. Another tip is to use binoculars or a camera and wait while the sun is setting until the last moments before staring at it.

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