What is a Mirage?
A mirage is an optical illusion, or deception. It occurs especially in certain conditions of the air when the air is very hot. Sometimes in deserts there are spots called oases, where there is water, and, as there is water, there are also green trees and shade. We are told that sometimes travelers think they are coming to an oasis only a few miles away, where they can get water and shade; and then, as they travel on, it disappears. A great explorer once “discovered” and named a mountain which did not exist, but which he had seen as a mirage.
But a true mirage is not an appearance in the sky due to nothing at all, and it is not pure imagination on the part of those who see it. When the traveler sees an oasis in the desert, and it fades and deceives him, what he has seen is the image of a real oasis, much farther on, below the horizon. The light from the real oasis has been reflected from a layer of air, and the traveler sees it as if there were a huge mirror in the sky placed at such an angle as to give a view of the oasis to the traveler’s eyes. The oasis itself may be many weary miles distant.
The reason for this is that there are layers of air of different temperatures, and therefore of different density, and whenever light passes from one thing into another of different density, part of it does not go on, but is deflected back. Appearances due to a similar cause are often seen at sea. A ship near the horizon may seem to have another ship, exactly like itself, perched upside down upon it.
