Alice and Bob are driving through the desert.

Alice: Looks dry.

Bob: That’s wrong, what we see ahead is caused by the sun heating up the road. While the speed of light is constant in vacuum, when light moves through matter, the atoms emit new light that destructively interferes with the old light, in effect causing a “delay”. This happens more with more atoms, meaning light travels slower through denser media.

Alice: OK, but—

Bob: So when the sun heats up the road, this creates a layer of thin warm air with denser cooler air higher up. As light passes through these layers, it refracts upwards towards the denser cooler layer. If conditions are right, this can bend the light back up towards your eyes. Does that make sense?

Alice: Yeah, but—

Bob: Now you might ask, “Why does this look like water?” The situations seem similar at first, with two fluids of different densities. But think about it: With an ocean, the dense water is below the thin air. While, in front of you, the dense cool air is above the thin warm air. The situations are actually reversed!

Alice: Please just—

Bob: With an ocean, there’s a sharp increase in density where the air meets the water. The Fresnel equations say that when light hits such a discontinuity at an angle, most is reflected off the surface. Whereas with the air in front of us, there’s a small and gradual decrease in density, meaning the light is slowly refracted through the lighter warm air and gradually bent back up. In both cases, the mixing in the fluids causes a shimmering effect.

Alice:

Bob: So yeah. Wrong. That’s just a heat mirage.

Meanwhile, the desert flies by, bone dry.