Green light at sunset.  If you stare at the sun anytime in the tropics and then immediately look at the sand, you will see the sun’s image reproduced on your closed eyelids, in your eye’s memory bank, even superimposed over what you are currently looking at, like some kind of falsely televised digital artifact.  This is a retinal image, and it always appears not in its original color, but in its complementary color, as if the eye had a memory bank in which it stored color charts, as indeed it does.  Thus the sun’s yellow is reproduced instead in its complementary color, which is green.

This may be what the green flash is: a contrarian optical reflex in our eyes when staring at an intense yellow-orange light (the sun), which then produces green when the sun disappears and is replaced by the suddenly darker sky against which our eyes produce a retinal memory of sun.

Goethe proved this many years ago (see my paper on Goethe’s color theory and Turner’s art).  

I’ve seen the green flash through sunglasses and through a car’s tinted glass which would have filtered it out had it been coming to me from the world, but in fact it is being created inside us by ourselves: like most miracles, it is man-made.

If produced by a distortion of the atmosphere or the bending of light or color in the thicker air lying at the visibly flat horizon of the oceanic tropics, then you would see the green flash in reverse, at dawn, before the sun comes up, as the air is similarly thick.  That never happens, and I’ve tried it on those bedraggled mornings when involuntary departure (voluntary in the winter weeks when reservations were made, before genuine reservations set in as the warm sun proved addictive and return became reluctant, a grim motion undertaken solely at the behest of moral obedience to the now forgotten self which booked, presumably more sanely, a physical retreat from a more hedonistic spiritual retreat), when departure forces the human spirit to sit disconsolately in the airports at ungodly hours, when the green flash is demurely mute because we haven’t been staring intensely at a too-bright yellow orb for a few minutes which then vanishes, but instead have been spacing out at a green sky, which then, were it to vanish suddenly and be replaced by some meek background blandness, would be reproduced in yellow, but of course the day doesn’t disappear with the sun, it grows brighter and drowns out such delicate ocular phenomena - only the sun vanishes with the evening, leaving us alone with our mysterious spasms and our inner eyes.