What is a mirage and why does a pond appear in the distance on a hot highway and then disappear before you reach it?

The mirage is actually a thin layer of hot air close to the surface of the road, heated by sunrays striking the ground. The layer need only be a few millimeters thick for a mirage to occur. Light travels faster in warm air than in cold, which has greater density, so any light rays that approach the hot layer at a low angle will be bent back up into the colder air above. The shimmering appearance of this bent light looks like the reflecting surface of a pond. The conditions most likely to produce amirage in the desert would be a layer of hot air lying immediately above the ground with cooler air above it (this is quite usual during the day because the ground becomes so hot). Light rays from a distant object, e.g. a tree, would travel in a straight line through the cool air to an observer’s eye. But other light rays from the tree would travel towards the ground and come in contact with the surface of separation between the cool and the hot air with their different optical densities. The rays which strike this surface very obliquely (at an angle greater than the critical angle) would be reflected upwards again and thus reach the observer’s eye. In this way the observer would see the distant tree not only upright but also inverted, as though mirrored in a pool of water. One can see a similar effect on heated roads during the summer. The reflection of the sky and clouds appears just above the surface of the road as though mirrored in a pool of water. This is known as an inferior mirage. As you approach the mirage, it begins to narrow and will ultimately vanish. This is because the angle of your line of vision becomes greater, whereas the hot air bends the light upward at only a slight angle--not enough to make you see a reflection. More spectacular mirages are the type sometimes seen at sea. They may take the form of a ship floating in the air, or the lights of a distant city shining in the sky. The conditions needed to produce this kind of mirage (superior mirage) are exactly opposite to those needed in the previous case. It requires a thick layer of cold air lying above the surface of the sea with warmer air above it. The light rays

from the ship which travel upwards and strike the surface of separation between the cold and warm air at an angle greater than the critical angle are reflected back into the eyes of the distant observer. In this way an observer may see a ship mirrored in the sky, though the actual ship may be out of sight, hidden by the curve of the Earth.