Dense air-strata.
The rays of light that come from the sun to the earth appear to travel in a straight line, but they never do. As soon as they meet with and pass into the atmospheric envelope they are bent or deflected from their original direction and reach the earth by obtuse angles or in long descending curves like a spent rifle ball. This bending of the rays is called refraction, which must not be confounded with reflection—a something quite different. Now refraction is, of course, the greatest where the atmosphere is the densest. The thicker the air the more acute the bending of the light-ray. Hence the thick layers of air lying along or a few feet above the surface of the earth on a hot day are peculiarly well-fitted to distort the light-ray, and consequently well-fitted to produce the effect of mirage. These layers of air are of varying densities. Some are thicker than others; and in this respect the atmosphere bears a resemblance to an ordinary photographic or telescopic lens. Let us use the lens illustration for a moment and perhaps it will aid comprehension of the subject.
Illustration of camera lens.
You know that the lens, like the air, is of varying thicknesses or densities, and you know that in the ordinary camera the rays of light, passing through the upper part of the lens, are refracted or bent toward the perpendicular so that they reach the ground-glass “finder” at the bottom; and that the rays passing through the lower part of the lens go to the top of the “finder” The result is that you have on the “finder” or the negative something reversed—things upside down. That, so far as the reversed image goes, is precisely the case in mirage. The air-layers act as a lens and bend the light-rays so that when seen in our “finder”—the eye—the bottom of a tree, for example, goes to the top and the top goes to the bottom.
The bent light-ray.
But there is something more to mirage than this reversed image. The eyes do not see things “in their place,” but see them hanging in the air as in the case of ships and caravans. To explain this, in the absence of a diagram, we shall have to take up another illustration. Suppose a light-ray so violently bent by the heat lying above a sidewalk that it should come to us around a street corner, and thereby we should see a man coming up a side street that lies at right angles to us. He would appear to our eyes to be coming up, not the side street, but the street we are standing in. The man, to all appearances, would not be “in his place.” We should see him where he is not.
Ships at sea.
Ships upside down.
Now suppose again instead of the light-rays bending to right or left (as in the street-corner illustration), we consider them as bending skyward or earthward. Suppose yourself at sea and that you are looking up into the sky above the horizon. You see there a ship “out of its place,” hanging in the air in an impossible manner—something which is equivalent, or at least analogous, to looking down the street and seeing the image of the man around the corner. You are looking straight into the sky, yet seeing a ship below the verge. The light-rays coming from the ship on the water describe an obtuse angle or curve in reaching the eye. The rays from the bottom of the ship, lying in a dense part of the air-lens, are more acutely bent than those from the masts, and hence they go to the top of the photographic plate or your field of vision, whereas the rays from the ship’s masts, being in a thinner atmosphere, are less violently bent, and thus go to the bottom of your field of vision. The result is the ship high in air above the horizon-line and upside down.
Wherein the illusion.