CBSE Class 10 Answered

A mirage is a naturally-occurring optical phenomenon, in which light rays are bent to produce a displaced image of distant objects or the sky
The principal physical cause of a mirage, however, is refraction rather than reflection.

Cold air is denser than warm air, and has therefore a greater refractive index. As light passes from colder air to warmer air it bends away from normal  a light ray coming from the sky toward the hot ground. If the air near the ground is warmer than that higher up, the light ray bends in a concave upwards trajectory. Once the ray reaches the viewer’s eye, the eye traces it as the line of sight, which is the line tangent to the path the ray takes at the point it reaches the eye. The result is that an inferior image for the above sky appears on the ground