What’s a green flash ?

A green flash is an optical phenomenon, such as a mirage, but it is not an optical illusion. You can see it at sunset or sunrise. It is a sudden moment (it does not last more than one second or two), when a part of the sun suddenly changes color from red/orange to green/blue so that’s why we describe it as a flash due to the ability of the air to separate the colors of the light. It is a rare phenomenon, and we can also observe it with the moon. It usually occurs on the horizon, when it is unobstructed : if there are clouds, ships, haze or anything that can avoid you to clearly see the horizon line.
It is so sudden that when I had the chance to witness one, I didn’t see it, actually I was taking photos of a sunset in Mauritius and it is only when I watched my photo afterwards that I realized I had caught a green flash.

The different kinds of green flashes

If you project the telescopic image of a the sun ready to set, about 10 minutes before it disappears, you will be able to see the upper and lower limbs of the sun (in astronomy, the limb is the apparent contour of a star). The lower half of the limb will appear red, and the upper half of the limb will appear green. But it’s difficult to see it, because even in the best conditions, the green and red limbs would be very tiny. Thus, it cannot be seen with the naked eye. This phenomenon is named the green and red rims (rim is usually used for the outer edge of car wheel or the edge of an object).
However, there is a unique phenomenon allowing us to see a green rim flash with the naked eye : the elevated horizon flash. It can be seen when the lower rim is occulted by an elevated obstacle such as a mountain, or even a skyscraper. In that way, the upper rim is isolated, and we can see it. Furthermore, it has a chance to appear blue or violet.
The green flash commonly appears in two forms : as a flash, when the green spot is rather extended, or as a dot.

Why is the green flash green ?

Firstly, green flashes can also be yellow, blue, violet, etc, but green is the color we most often see. The atmospheric dispersion is the cause for the color. Indeed, the refraction changes a bit according to the color of the light, and dispersion is the variation of the refractive index with the wavelength of those colors of the light. It is a refractive property of all transparent media : they all are dispersive. And so refraction by the air is larger at shorter wavelength. And then, while the sun sets, the refractive delay of the colors is usually longer for blue (wavelength : between 430 and 380 nm) and violet (wavelength : between 500 and 430 nm) than for red (wavelength : between 740 and 625 nm).
In fact, the colored images of the sun disappear from red to violet (red-yellow-green-blue-violet). Dispersion is usually small enough to be negligible, and it is visible only in special situations.
However, the green flash should be violet, because it is supposed to be the last color seen at sunset. So why do we see it green ? Here is another phenomenon in cause : the atmospheric extinction, or how the wavelength are scattered. When light passes through a medium not perfectly homogenous, such as air, the light is scattered in all directions by the medium’s irregularities. Air molecules are large enough to scatter the light, and the shortest wavelengths are scattered most strongly (the best example is the blue color of the sky : blue light has got a short wavelength, so is it strongly scattered, and the blue rays go in all directions, therefore we see the sky as blue).
On the horizon, the path from the air to the observer’s eye is very long, so when the light comes toward your eye, the shortest wavelengths are almost completely removed. And usually, there is enough haze in the air to totally remove the violet and blue colors, so that the last color to be seen is green, or, if the green flash occurs at sunrise, green would be the first color to be seen.
Even the yellow color disappears when you see a green flash. If you look at the sunset, the red-sensitive receptors of the retina would be affected, and for a while, the color perception is slightly changed without you being aware of it. It comes out as seeing the yellow part of the visible light at sunset as green. This effect can be avoided by observing green flashes at sunrise, but it is difficult to guess where the sun is going to rise before it’s there.