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What causes a mirage?

What causes a mirage?

Reeta Vyas, professor of physics in the J. William Fulbright College of Arts and Sciences, replies:

Mirage is an optical phenomenon, which can be explained in terms of refraction of light and variation of air density with height in the atmosphere.

A light ray entering a thinner medium from a denser medium is refracted or deflected toward the interface between the media. On a hot sunny day, the warmer air nearthe earth’s surface is less dense than the cooler layersabove it. The density of air thus increases with height.Under these conditions, a ray from an object, say a tree,traveling towards the earth, keeps bending toward the interface as it passes through different layers and is eventually reflected back in the upward direction.

To a person seeing this reflected ray, it appears to come from a point below the ground. Thus the person sees an inverted image of the tree below the tree creating the illusion of reflection in water. This is called inferior mirage because the image is formed below the object. In cooler climates, the air near the earth’s surface is denser than the layers of air above it causing the density of air to decrease with height.

A ray traveling upward from an object, say, a ship, is refracted towards the ground creating an inverted image of the ship in the sky. Since the image is formed above the object it is called superior mirage. More complex variation of air density can lead to more intricate image patterns known as Fata Morgana,or Fairy Morgan, after the legendary shape-changing half-sister of King Arthur.

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