Some of these are milder versions of the phenomena that produce mirages. .
Mirages are phenomena of atmospheric refraction; so to understand mirages, you first have to understand refraction in the atmosphere. .
The air near sea level is about a thousand times less dense than water; but that's still enough to change the direction of light rays that enter it from a different medium, such as the nearly empty space outside our atmosphere. And, because the density of the air changes continuously with height above the Earth, light rays within the Earth's atmosphere bend continuously as they pass from one level to another. Usually, the density decreases steadily from the ground up, so the rays (which always bend toward the denser material) are concave toward the surface of the Earth. .
Also, if an object is to appear inverted, rays from the top of the object must cross rays from the bottom of the object on their way to the observer's eye, so that the former arrive at the eye below the latter. .
There are several different kinds of mirage. Each involves a particular thermal structure in the atmosphere, and a particular relative placement of the structure and the observer. Here are some common examples: .
The Inferior Mirage.
The commonest mirage is the "inferior" mirage, so called not because it is a poor example, but because the inverted image lies below the erect one. This is sometimes called the mirage of the desert, but today it is more appropriately known as the "hot-road" mirage, as it is seen on asphalt paving nearly every sunny day.
The illusion part of the mirage generally comes from our mind's interpretation of what the eye sees. When our eye sees a light ray coming from an object, our mind interprets the ray path as a perfectly straight line from the object to our eye. Only when we recognize that we are seeing an illusion can we make some mental corrections to the scene. But recall what happens initially when we are surprised by such a scene.