# What causes a mirage?

Edwin Meyer, a physics professor at Baldwin-Wallace College, explains.

To understand how a mirage forms, one must first understand how light travels through air. If the air is all the same temperature--cold or hot--light travels through it in a straight line. If a steady temperature gradient exists, however, light will follow a curved path toward the cooler air. The standard freshman physics explanation for this phenomenon is that cold air has a higher index of refraction than warm air does. As a result, photons (particles of light) travel through hot air faster than they can through cold air because the hot air is less dense. The quantum electrodynamics explanation is that photons always take the path of minimum time when traveling from one point to another. In order to get from one point to another in a minimum time, photons will take "shortcuts" even though the length of the path is curved and it covers a longer distance than the direct route.

Mirages are a direct result of photons taking the path of minimum time in vertical temperature gradients. Ideal conditions for a mirage are still air on a hot, sunny day over a flat surface that will absorb the sun's energy and become quite hot. When these conditions exist, the air closest to the surface is hottest and least dense and the air density gradually increases with height. Incoming photons take a curved path from the sky to the viewer's eye. The illusion comes from the fact that quantum electrodynamics is not intuitive and the human brain assumes that light travels in a straight line. A viewer looking at, say, the road ahead on a hot, still, day will see the sky because photons from the sky are taking the curved path that minimizes the time taken. The brain interprets this as water on the road because water would reflect light from the sky in much the same way that a vertical temperature gradient does.

A simple experiment can demonstrate the manner in which a light beam bends in a vertical density gradient. Fill a long glass tank with water, dissolve sugar in the water and shine a laser beam in one end. The vertical gradient produced by the sugar concentration will cause the beam to bend. If the tank is long enough and a mirror is placed on the bottom, the beam will "bounce" along the bottom of the tank.

Originally published on November 17, 2003.

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1. 1. mickeyflippindougal 06:47 AM 5/14/10

So does this mean that "the shortest distance between two points is a straight line" is not always true?

2. 2. rajeev2593019 06:38 AM 9/4/10

as the light bends away from normal before bending down it must become perpendicular to normal , than there should not be any further change in path. why then it bends down?

3. 3. tribbledibblebibble6 in reply to mickeyflippindougal 11:05 PM 3/11/11

No.

4. 4. Yo322 03:52 AM 5/10/12

To understand mirage it's as simple as placing multiple sunglass lens and looking through it. The image becomes darker the more lens are placed. Now you use that concept and add osmosis, humidity and evaporation. Osmosis explains how water behaves; humidity is airborne water thus the rules of osmosis applies to humidity; and evaporation occurs when water hits a certain temp for example the hot surface of a road( or the hot sands of the desert. Now the mirage is formed because the rate (speed of the rising water vapor) of evaporation remains constant while the rate of osmosis varies (as humidity touches the hot surface of a road it evaporates which causes humidity to rush in to keep osmosis true). Now add the sunglass lens concept with the water vapor that's being produce by humidity and the illusion of water appearing on the road will exist. As for the sugar bending light to bend down could be explain by the position of the sugar molecules in water reacting to gravity and how the elements in sugar react to light.

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