When you're driving through the desert and it's hot, you sometimes see what looks like a big pool of water in the road ahead of you. I know this is a mirage and that it's not really water. But sometimes there's a reflection of a car or a house or something in the mirage. How can there be a reflection when the mirage isn't really water?
-- Wondering, El Cajon
What you're seeing is called an "inferior" mirage, located below your line of sight. A "superior" mirage? Say, a Starbucks floating through the sky above a glacier on a hot day. The store itself might be 5 or 25 miles over the horizon, but atmospheric density bends light rays and can create some strange effects. A mirage is an actual image, not something your heat-addled brain cooked up. You can whip out the Instamatic the next time you see one, click away, then bore your friends senseless with vacation pictures of upside-down trucks coming at you on the freeway.
The typical mirage on hot asphalt or sand is caused when light rays pass from cooler air into the thin layer of less dense, very hot air at ground level. When light moves from a medium of one density to a medium of another density, the rays bend in predictable ways. When you're looking out at the vast nothingness in the desert, most of the light rays from all that monotony travel to your eyes unmolested. But a certain few of them will be coming in at just the right angle to bend (refract) downward as they enter the scorching air layer, then they reflect up into your eyes. If you see a pool of blue water on the freeway, what you're really seeing is an image of the sky; it's just in an illogically low place because of the refraction. Same thing with the truck or trees or whatever. They're not reflected in the "water" mirage, they are the mirage. The image is upside down because rays from the top of the object are bent at such an angle that they arrive at a lower point in your eye than the rays from the bottom of the object. Whew! Time for me to crawl over to that nice Roberto's I see on the horizon.