Abstract: Starting from the fact that the speed of light in a vacuum is the largest and the law of refraction ( or ray trace equation) ，it is proved that the initial angle α of the light ray being able to return to the ground in mirage can not be arbitrary. When α is greater than the certain value αm，the light emitted from a point is not able to return to the ground. The other factors affecting αm are also given.

Key words: mirage, atmospheric refraction, light ray, initial angle, refractivity