Mirage over Paris, France, seen between 3 and 4 in the morning in December 1869, historical artwork. This type of mirage, where the virtual object is seen above the real one, occurs when a layer of warm air lies above a cold layer of air. The warm air has a higher refractive index, and this bends light back towards the ground. Engraving from The Atmosphere (1873) by Camille Flammarion.