A mirage is no hallucination: you can photograph it
The shimmering pool on a hot road is not in your head. Air of different temperatures really bends light, so a camera records the false image too. Your brain supplies the story, reading a patch of sky as water. In 1596 the same physics made the Sun rise two weeks early for stranded Arctic sailors.
The word traces back through French to the Latin mirari, to look at or wonder at. Physically, a mirage happens when light passes through air layers of different density and is refracted, so a distant object or piece of sky appears somewhere it is not. What we believe we are seeing, though, depends on interpretation, which is why the classic desert illusion is water.
The everyday version is the inferior mirage, where the image sits below the real object. Sun-baked tarmac or sand can be more than 10 degrees Celsius hotter than the air a metre up, and light from the low sky bends upward through that gradient to reach the eye from the ground. The result is a bright, bluish, usually inverted patch that looks like a puddle and retreats as you approach, much as a rainbow does. Rising hot air mixes with sinking cool air, making the picture wobble, stretch or squash; the same heat shimmer shows above campfires and behind jet exhausts. Such images rarely exceed about half a degree in height, roughly the width of the Moon.
Flip the layers, with cold air beneath warm, and light bends downward instead, lifting images above the real object. These superior mirages are common over polar ice. When the bending matches Earth's curvature, light can travel from beyond the horizon, as the crew of a ship hunting the Northeast Passage discovered in 1596 when trapped at Novaya Zemlya: the Sun appeared while really still below the horizon. Faraway ships can appear to float in the sky, which may account for some explorers' tales of flying vessels and airborne cities.
The most elaborate kind is the Fata Morgana, named after the shape-shifting fairy half-sister of King Arthur, a rapidly changing stack of stretched, squashed, upright and upside-down images. Seeing one requires standing inside an atmospheric duct. Ludovic Kennedy described an alleged 1941 episode in which the pursued Bismarck seemed to charge at two British cruisers and then fade away, while radar showed it had not changed course.
Source: Mirage
