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Mirage from Selsey beach, southern England imaged by Mark Parrish April 6, '08 shortly after sunset, the Portsmouth skyline 23km away is seen apparently mirrored in the sea. ©Mark Parrish, shown with permission

The mirage, called an 'inferior mirage' because the image is below the real object, is produced by a layer of abnormally warm air heated by the sea and beneath cooler layers. Light passing at low angles across the layers is refracted so that rays coming from the tops of the building appear to be coming upwards from the horizon. Our brain interprets that as a reflection from water.

There is more. The horizon is missing and the base of buildings and low lying land cannot be seen either below or above the line joining the 'real' view and the mirage. This is a 'vanishing-line' effect.

The ray diagram at right tries to show what is happening but like all mirage ray diagrams the vertical scale has to be grossly exaggerated. All possible rays between the eye and a distant tower are shown. Two rays from the tower top (A) reach the eye. The upper one is slightly curved downwards towards the warmer air. The lower ray is sharply curved and appears to the eye to come from a tower reflected in water. Rays from lower down (B) do the same. Position (C) is different. Only one ray reaches the eye. Rays from lower down the tower cannot reach the eye at all - that part of the tower is invisible!

The level (C) is the level of the mirage "vanishing-line". The 'real' view and inverted mirage view join at the vanishing line and the eye can see no details of the scene that are below it. the more distant the object, the higher is the vanishing line

When the mirage is above the object it is called 'superior'. An extreme example, a Fata Morgana, is here. Mirages of the sun can produce a green flash.