Why Does Heat Seem to Make Things Quiver?
When a hot current of air passes between an object and our eyes, the object appears to quiver. It is only an appearance, however, and the phenomenon is one out of many illustrations of the fact that the eyes are very easily deceived. This is what happens. The air is a mixture of gases, and gas, like other things, expands when heated. It becomes lighter in density; the same bulk is now lighter.
When light is passing through a gas, it travels in a straight line as long as the gas remains of exactly the same density.
Whenever the density changes, light alters its course. Therefore, when hot currents come between an object and our eyes, the light coming from the object, and passing through the hot air which is continually changing in density, is bent this way and that. So it comes about that the eye, instead of seeing the object stationary, as it really is, sees it wobbling because the light is made to wobble by the changing temperature of the air.
Scientists term this bending of light, when it passes from one density to another, refraction. This refraction plays a large part in matters of sight. It is by taking advantage of this peculiarity of light that we are able to make all sorts of valuable optical instruments, including magnifying glasses, microscopes, telescopes, cameras, sextants and so on. All these are based on refraction, or light-bending.
