Desert mirage. Illustration showing how a mirage is produced. A mirage is an atmospheric phenomenon caused by the differential heating of layers of air. As air warms up, it expands, and this affects its refractive index. Light rays from the palm tree encounter a hot layer of air just above the ground and are refracted to a shallower angle. If the angle is sufficiently shallow, the light rays are refracted so that they appear to be reflected upwards. An inverted image of the tree is seen by the observer, as though it were reflected in water.