THE MIRAGE

It's 2:00 p.m. on a very hot afternoon, and we've just pulled out of the entrance to the express road. Far ahead of us a puddle seems to be continuously evaporating off the sun-baked pavement, its receding edge matching its speed to ours. Even when my old man floor the accelerator we can't catch up to the water, but somehow the cars passing ours easily do. They drive into it without a splash, and they seem to be reflected in it, upside down.

Of course, real water doesn't behave this way. The retreating puddle is just the most familiar form of that often misunderstood phenomenon of air and light, the mirage.
Is the puddle mirage real or is it an illusion? It's actually a little of both-straightforward physics plus the workings of the human mind. Add a little dehydration, which we almost always experience on hot days like this, and perhaps we're even more inclined to see water where it isn't! Scientists relax and read.

To understand how a mirage forms, one must first understand how light travels through air. If the air is all the same temperature--cold or hot--light travels through it in a straight line. If a steady temperature gradient exists, however, light will follow a curved path toward the cooler air. The standard freshman physics explanation for this phenomenon is that cold air has a higher index of refraction than warm air does. As a result, photons (particles of light) travel through hot air faster than they can through cold air because the hot air is less dense.

The quantum electrodynamics explanation is that photons always take the path of minimum time when traveling from one point to another. In order to get from one point to another in a minimum time, photons will take "shortcuts" even though the length of the path is curved and it covers a longer distance than the direct route. Mirages are a direct result of photons taking the path of minimum time in vertical temperature gradients. Ideal conditions for a mirage are still air on a hot, sunny day over a flat surface that will absorb the sun's energy and become quite hot. When these conditions exist, the air closest to the surface is hottest and least dense and the air density gradually increases with height. Incoming photons take a curved path from the sky to the viewer's eye. The illusion comes from the fact that quantum electrodynamics is not intuitive and the human brain assumes that light travels in a straight line. A viewer looking at, say, the road ahead on a hot, still, day will see the sky because photons from the sky are taking the curved path that minimizes the time taken. The brain interprets this as water on the road because water would reflect light from the sky in much the same way that a vertical temperature gradient does.