SHEILA TERRY / SCIENCE PHOTO LIBRARY SHEILA TERRY / SCIENCE PHOTO LIBRARY
Mirage over Paris, coloured historical engraving. This mirage was seen between 3am and 4am in December 1869. 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.
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