|MadSci Network: Earth Sciences|
Wendy, Thanks for the question and sorry about the delay. Mirages have to do with there being different densities of air in layers next to each other. The air density is important because where there is a change in density the path of light rays will bend. This is known as refraction, and applies in other situations besides mirages, like when light follows a path from air to water. On common way to get layers of air with different temperatures is when there is a difference in temperature between two layers of air. There are two cases or types; where cold air is above warm air, and when warm air is above cold air. The mirage that you witnessed is of the first type. There is another kind of mirage sometimes called "Fata Morgana" that is of the second type. During a hot and sunny day, the black or gray surface of a highway gets much hotter than the air above it since it can absorb sunlight much better than the transparent air. The hot highway heats the air directly above it by conduction and convection. This air becomes hotter than the air that is above it and the conditions for a mirage are met. Look at the first diagram I have included. It shows a light ray coming down from the left and then reaching the layer of hot air near the road. As it reaches this hot layer it bends toward the cooler and denser air above. If it does not have to bend too much it will reverse its vertical direction and start going up away from the road. While the light has followed a curved path, it looks just like it has reflected off of a mirror on the road. The second diagram tries to put the first one into context. A person on the left is standing on a hot highway. An observer on the right has to look down toward the road to see the mirage/reflection of the person. Notice that the reflection will appear upside down. I hope this answers your question. Regards, Everett Rubel
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