What makes us see mirages?

Not that I’ve actually seen one…but today was hot and I noticed that I could see heat waves with the naked eye? How come …and Y do people see mirages?

It’s courtesy of Schnell’s Law, a fascinating little law describing the phenomenon that makes fiber optics (among other things) possible. No doubt some optics expert will be along to outclass me soon, but here’s the phenomenon as I understand it:

At the interface between two materials (or sections of one material, like air) with different densities, there’s an angle at which light will reflect off the less dense one and back into the denser one. Try looking at the water surface in a fish tank from just below the waterline, and you’ll see that it looks like a ripply mirror; if you look at it from a greater angle, you can see through it.

A mirage results in a special case of this effect. At some location (often above a body of water), the air is much cooler (and therefore more dense) than a layer of air high above it. The image of whatever is in that location reflects of off the warmer, less dense layer. Sometimes this is visible directly as a floating image, either at the horizon or high in the air, to someone at just the proper angle to see it. Sometimes it reflects again off a similar “mirror” at ground level(the hot ground produces a shallow layer of thin air–the same effect that makes “highway shimmer”) and appears to be a feature of the land ahead, even though it may be many miles away.