Mirage

A mirage is a naturally-occurring optical phenomenon, in which light rays are bent to produce a displaced image of distant objects or the sky. The word comes to English via the French mirage, from the Latin mirare, meaning 'to look at, to wonder at'. This is the same root as for mirror and to admire. Like a mirror, a mirage shows images of things which are elsewhere. The principal physical cause of a mirage, however, is refraction rather than reflection. A mirage is a real optical phenomenon that can be captured on camera, since light rays actually are refracted to form the false image at the observer's location. The interpretation of the image, however, is up to the fantasy of the human mind, and is easily mistaken for a small body of water.

Cold air is denser than warm air, and has therefore a greater refractive index. As light passes from colder air to warmer air it bends away from the direction of the temperature gradient (the "normal" in the figure on the left); when it passes from hotter to colder, it bends towards the direction of the gradient. The diagram on the left shows a light ray coming from the sky toward the hot ground. If the air near the ground is warmer than that higher up, the light ray bends in a concave upwards trajectory. Once the ray reaches the viewer’s eye, the eye traces it as the line of sight, which is the line tangent to the path the ray takes at the point it reaches the eye. The result is that an inferior imagesuperior image.

The 'rest' state of the Earth's atmosphere is with a vertical gradient of about -1 degree Celsius per 100 metres height. (The value is negative because it gets colder when you go higher.) For an actual mirage to happen, the temperature gradient has to be much greater. According to Minnaert the magnitude of the gradient should be at least 2°C per meter, and the mirage will not get strong until the magnitude reaches 4 or 5°C per meter. These conditions can occur when there is strong heating at ground level, for example when the sun is shining on sand or asphalt.

SOURCE : http://en.wikipedia.org/wiki/Mirage 

edit @ 26 Nov 2008 21:40:54 by hikari505

edit @ 26 Nov 2008 21:42:10 by hikari505