Green Flash at Sunset?

I received this question in an email, and thought I’d share my response with everyone, because it is such a cool thing to see (the flash, I mean… not my attempt to answer the question).

The green flash is very real, but it’s hard to see because you need very special conditions (like a very clear day at the Pacific or the Sahara) in order for it to be visible. Did you know that it can be blue or purple (violet) also? The human eye is tuned for green, so that’s the one we tend to see more easily. The green flash is usually a ray of green light firing vertically above the setting sun, although it can also be seen as a green band circling over the setting sun.

Why does it do that? You see a green flash because the sunlight rays refract (bend) when they zip through the air. But they bend in different amounts – violet bends the most, while red bends the least. At sunset, when the sun is at a very low angle, the sunlight is split into its spectrum of colors: red, orange, yellow, green, blue, and violet. The red, orange, and yellow light are the first to disappear (absorbed by the atmosphere), leaving the violet light to be last one visible… but blue and violet light are often scattered/absorbed by the atmosphere, so the last light left is green. (This effect only happens for a few short seconds, so we call it a ‘flash’.)