Sima Qian's "Historical Records of the Western Han Dynasty Tianguan": "The seaside meteorological tower, wild gas into a palace." A mirage is an optical illusion, a virtual image formed by the refraction of light reflected by objects on the earth through the atmosphere. Because different air layers have different densities, light has different refractive indexes in air with different densities. That is, because the density between warm air on the sea surface and cold air in the high air is different, it refracts light.