Why do we see green flashes during sunset?

The last bit of sun you see as it's going under the horizon is actually when the sun is already under the horizon, and what you're seeing is light being bent by the atmosphere, a bit like light is in prism and it's actually bent down towards you. Now, red light is bent less than green light which is bent less than blue light. And so, as the sun goes down, the very, very last light you should be able to see would be blue and then just a bit before that you'd see green, and the bit before that you see yellow and red. But normally, the atmosphere is too dirty if you're to ever see the blue because blue gets scattered out because the sky is blue so the blue is just isn't there and also normally, the atmosphere is too dirty to see the green. But if you happen to be on an incredibly, incredibly clean atmosphere, it normally happens out at sea because there's less dust in the atmosphere out at sea, because there's less dust there. And if it's been very, very calm, you can just sometimes see that green light being refracted around by the atmosphere and you get this thing called a green flash and it's incredibly rare. I've known somebody who's seen it a couple of times.