SciTech

Varying air temperatures cause mirages

Credit: Brooke Kuei/SciTech Editor Credit: Brooke Kuei/SciTech Editor

Have you ever seen puddles on the road while driving only to see them disappear moments later? Have you ever seen a sun on the horizon that looked like it was both rising and setting? These seemingly impossible occurrences are actually the result of a phenomenon known as a mirage. Mirages are caused by atmospheric conditions and contain at least one inverted image of an object.

Generally, mirages are the result of refraction. Refraction occurs when a light wave crosses the boundary between two mediums at an angle. Part of the light wave will speed up or slow down before the rest of the wave. This change in speed causes the light wave to change direction or bend.

In the case of mirages, two different mediums are formed by a distinct, sudden change in air density in the atmosphere, according to howstuffworks.com. The most common cause of varying air density is varying air temperature, since hot air is less dense than cold air. One example of this temperature difference involves an asphalt road on a hot day. The asphalt, after sitting in the sun for an extended period of time, heats the air directly above it to a higher temperature than the rest of the air in the atmosphere. The change in density between the hot air near the road and the cooler air higher in the atmosphere causes light rays to bend as they move between the mediums.

This bending of light causes mirages by affecting the way in which our eyes perceive the light reflecting off an object.
Normally, light only travels in a straight line. The light bouncing off an object reflects in all directions. Your eyes perceive the light that bounces off the object straight toward you, and you see the object. When light wave bending occurs, however, your eyes detect additional light waves. Light still bounces off the object in all directions, so you still detect light waves that reflect straight toward you, and you still see the object. However, your eyes can also detect bent light waves, which cause you to see a mirror image of the object.

In the instance of a mirror image of a car on a highway, the light waves that would normally simply reflect toward the ground hit the hot layer of air near the asphalt and are bent upward toward your eyes. According to howstuffworks.com, the light waves reflected by the bottom of the car hit the hot air first, speed up, and are bent farther upward than the light waves reflected from the top of the car. Because the lower light waves are bent farther upward, your eyes perceive the lower part of the car to be above the top of the car. This phenomenon causes your eyes to see both the original car and a mirror reflection of the car below the original. To a viewer, this type of mirage often looks like a puddle of water, since it reflects everything located above it.

Since the mirror image in the previous example is below the horizon, the mirage is labeled as an inferior mirage. Mirages can also be superior mirages, which form above the horizon. Superior mirages typically occur when there is cold air underneath warm air. Superior mirages occur in the same way as inferior mirages, except that the light is bent in the opposite direction, which causes the flipped image to appear on top of the original image, instead of underneath.

Mirages, despite their reputation as highly irrational visions, are the result of the simple, physical concept of light refraction and can lead to a variety of different illusions and produce complex reflections apparent in everyday life.