The green flashes seen over a land horizon are proof enough that the sea is not necessary to produce a green flash, and that at least some flashes are purely atmospheric. But this does not rule out the idea that the ocean itself plays a part in those flashes that are seen at a sea horizon.
A sea horizon favors green flashes in two main ways: the water's heat capacity is responsible for thermal structure in the adjacent air that causes mirages and their associated flashes; and the sea horizon lies below the astronomical one, which is a requirement for mirages of the Sun to be seen.
However, the mistaken notion that green flashes close to the visible sea horizon are caused by the Sun “shining through the waves” is so persistent that some detailed counter-arguments seem needed. Here's an attempt to provide them.
First of all, as the flash appears just at the sea horizon where the Sun is disappearing at sunset, whatever the Sun is shining through does not bend its rays appreciably differently than does the air though which the Sun was seen before the flash. If we forget about atmospheric refraction for the moment, that means the waves would have to produce no deviation of the Sun's rays. That means the path of the supposed ray through the wave would have to look something like this cartoon:
Here the ray from the Sun would have to just punch through the wave without being deflected.
But that isn't how light behaves! When light passes from air into water, it's always bent toward a line drawn perpendicular to the air-water surface at the point of entry. Let's draw that line, and see where the light goes:
Here, the dashed line is perpendicular to the wave surface where the light enters the water. The light bends toward this dashed line — see the refraction page for the details — and so is bent down, into the ocean, as it enters the wave. Once the light gets into the water, it can't get back out. (Even if it were to encounter the far side of the wave, it would get bent down there again, because the normal to the surface tilts the other way there, and the ray bends away from the normal on passing from water to air.)
If the sunlight were really to “shine through the waves,” those waves would have to have vertical sides, like panes of window glass. But waves on the ocean don't have vertical sides. They're more nearly horizontal; the slopes of waves (except where they break, near the shore) rarely exceed 15 or 20 degrees. (The waves in the cartoons above are greatly exaggerated in steepness; real waves are much flatter.)
If the water surface were flat, the light would be bent down even more sharply. So the waves do allow some of the refracted rays to descend less steeply than they would if the water were completely calm. But, unless the waves are absolutely vertical, the light that enters them is always bent downward and lost. The flatter the surface, the more steeply the rays of light are bent downward.
Another way of looking at the optics of the water surface is from the point of view of a fish, or a diver under water. If you open your eyes under water (swimming pools work better than the ocean, here, as the water is clearer) and look up, you'll see that the whole hemisphere of sky above the water is imaged into a cone with a vertex angle a little larger than 90°. The horizon of the world above water appears well above the horizontal under water. If we forget about waves for a moment, and just have a calm, flat water surface, it looks something like this:
Here the solid rays show how the whole 180° of sky above the water is imaged into a much smaller angle below the water. The remaining directions in the water, below the image of the horizon, are rays that are totally reflected back into the water, as shown by the dashed lines.
Note that any ray making an angle less than about 40° with the water surface from below is totally reflected. To a fish, or a diver, the water surface looks like a perfect mirror, except for the “hole” directly overhead in which the entire sky is imaged.
This means that even if a ray of sunlight could somehow get to be horizontal inside the water, instead of sloping downward, it could never get out through the surface. And remember that wave slopes are usually less than 20 degrees; so even with a fairly steep wave, a horizontal ray under water is trapped there, and can never get into the air.
So the Sun's rays can't shine through waves, and can't get out of the water to be seen by a green-flash observer once they get below the water surface. Green flashes can't be due to sunlight passing through water at all.
There are other problems with transmission through water as well. Drop a white card into the sea and notice its color and brightness. The color is pale at first, and becomes a more saturated green as you look through a greater depth of water. But it only becomes moderately saturated when the loss of brightness is considerable. And it never gets to be as saturated a green as a green flash.
But green flashes are usually described as “brilliant” or “emerald green”, not the muddy green of submerged white objects. Sea water isn't a clear green filter; it's murky, and when you see through enough of it to be really green, fairly dark.
Besides, if you look at green flashes carefully, you find they run through the hues of the spectrum: at sunset, from yellow-green through green to blue, and sometimes even to violet. This is the hallmark of dispersion, not an absorbing filter.
One reader has suggested to me that the sunlight that enters the water might be scattered back out by the particulate matter suspended in the water. Indeed, some of it is; but it is scattered in all directions, and so produces a very low-brightness background, not the brilliance of a green flash. In fact, it is this scattered light that is responsible for much of the apparent brightness of the sea — which is usually much less than the brightness of the sky (another result of scattering in all directions, but in air rather than water).
The optics of sea water do not allow green flashes to be explained by light transmitted through sea water, either directly or after scattering by suspended particles.
Sea water's role in producing green flashes is entirely thermal, not optical. The sea's heat capacity produces the strong thermal contrasts in the surface layers of air that are responsible for mirages and green flashes.
Copyright © 2004 – 2006, 2012 Andrew T. Young
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