In cartoons, a mirage elaborates visions of tropical oases, complete with palm trees and ornate swimming pools; they spring up suddenly in the hot desert, and then disappear just as the hero is about to dive in. This sort of illusion is complete fiction, of course; but mirages do really exist, and they can make you see water where there is not any.
In the most common types of mirage, an object appears to be reflected as if there were a pool of water on the ground. This phenomenon is caused by light refraction—the bending of light beams when it passes from one medium into another. Imagine you are pushing a shopping cart across a parking lot; if you are exerting a constant force, the cart’s speed depends on the medium it is traveling on—in this case, the parking lot’s paved surface. What happens when you push the shopping cart out of the parking lot onto a grassy area?
If you push it straight onto the grass, it will simply slow down; the grass medium offers more resistance, so it takes more energy to move the shopping cart. If you push the cart onto the grass at an angle, something else happens; if the right wheel hits the grass first, it will slow down while the left wheel is still on the pavement; because the left wheel is briefly moving more quickly than the right wheel, the shopping cart will turn to the right as it moves onto the grass. Similarly, if you move from a grassy area to a paved area, one wheel will speed up before the other and the cart will turn.
A light wave works in a similar way; its speed depends on what sort of medium it is passing through. In the vacuum of space, light travels at top speed because there is not any matter slowing it down. It has a harder time moving through an area filled with matter, such as the Earth’s gaseous atmosphere, so it moves more slowly. As it moves from one medium to another at an angle, one part of the wave changes speed an instant before the other, and the light turns.
Mirages occur when there is a rapid shift in air density in the atmosphere while the air at one level is a lot hotter than the air at an adjoining level. This commonly occurs on summer days, when an asphalt road that has been baking in the Sun heats the air directly above it, creating a sharp shift in air density levels near the ground. As light passes between the different levels, it bends, creating mirages.
Normally, sunlight bouncing off an object—let us say a car—reflects in all directions; you see the car when your eyes detect this light. On an cloudy day, you only see the light that bounces off the car straight toward you; this is how you see things most of the time. On a sunnier day, the light heading straight toward you acts just like it usually does; it does not move through different layers of air density, so it does not bend much.
Some of the light that would normally hit the ground actually bends in midair because it moves from the cooler, denser air level into the hotter, less dense air right above the ground. The lower part of the light wave passes between the layers first, so it speeds up an instant before the upper part; the light that would ordinarily go straight to the ground bends upward and travels to your eyes. The effect is that you see the image of the car twice; once on top of the road, and once on the road surface.
The light from the lower part of the car bends farther upward than the light from the top of the car, so the mirage image looks like a reflection. Your brain assumes that the light is traveling in a straight line, so it seems like there is a mirror image beneath the normal image. This mirage looks just like a puddle of water on the road because, like a puddle of water, it is reflecting what is above it. This sort of mirage is known as “an inferior mirage” because it appears below the horizon.
“Superior mirages” are mirages that form above the horizon. This occurs when there is a cooler level of air lower than a warmer level of air, typically over icy landscapes or very cold water. This mirage causes you to see a scene much higher than it should be; for example, you might see a mass of land or a boat floating in midair. This situation might also distort images, making a boat seem much taller than it actually is.
You can see a similar optical illusion any day when the sky is clear. As light from the Sun enters our atmosphere, it slows down considerably; as a result, it bends toward the Earth. When the Sun is low in the sky, it appears to be higher than it actually is because of this refraction. When it looks like the Sun is about to drop below the horizon, it already has; the atmosphere is bending the light around the curve of the globe.
References
http://science.howstuffworks.com/
http://www.physics.org/
Cover image: canva.com