When the sun sets, one can sometimes see a green flash, just for a second or two. It’s the effect of refraction. The sun is just about to go below the horizon, and its light rays get “bent” by the atmosphere, splitting into colours like through a prism. The blue colour is “bent” the most, but gets scattered by the air, and so green remains visible (red and orange, undergoing the least refraction, go up into the sky, obviously).
I’ll try to shoot a better picture, but you get the idea. Sorry it’s so small, but my longest lens is 200mm; yes, I cropped maximally for the resolution of the enlarged picture (longer side 1024 pixels).