Green ray

The Green Ray or Green Flash occures shortly after sunset or even at dawn. Just as the sun sets/rises, a green ray shoots up. This is a rare optical phenomenon.

Its explanation lies in refraction of light in the atmosphere and is enhanced by atmospheric layering. It is usually observed from a low altitude where there is an unobstructed view of the horizon, such as on the ocean. Whilst we would expect to see a blue light, the blue is dispersed (this is why the sky is blue)and the green flash is visible from a fraction of a second to a couple of seconds duration.

With slight magnification the effect can be see on the top limb of the solar disk on most clear-day sunsets. However the flash effect requires a stronger layering of the atmosphere.

External links:

green flash (http://www.weather-photography.com/Photos/gallery.php?cat=optics&subcat=green_flash)