mirage

An image formed under certain atmospheric conditions, in which objects appear to be reflected or displaced or in which nonexistent objects seem to appear. For example, the difference in the index of refraction between a low layer of very hot air and a higher level of cold air can cause light rays, travelling down from an object (such as the sky or a cloud) and passing through ever warmer air, to be refracted back up again. An observer viewing these light rays perceives them coming up off the ground, and thus sees the inverted image of the object, which appears lower than the object really is. In this way the sky itself can be reflected, resulting in the mirage of a distant lake.