SciTech

How Things Work: Mirages

Credit: Terry Brown, Art Staff Credit: Terry Brown, Art Staff

In 1939, Captain Bartlett of the Effie M. Morrissey witnessed a remarkable feature while traveling on the waters between Greenland and Iceland. To the southwest of Iceland appeared a glacier around 30 miles away. This appearance would not have been remarkable if the captain had not known that that very glacier, called the Snaefells Jökull glacier, was nearly 350 miles away. On sailing closer, the glacier suddenly seemed to disappear. The captain had seen what we now know to be a mirage. This account, reported in an article in the Alaska Science Forum, is now called the “Arctic mirage.”

The Arctic mirage is not the typical image that comes to one’s mind on hearing the word “mirage.” People usually tend to associate mirages with images of oases floating mysteriously in the midst of deserts. Cartoons usually embellish their oasis-mirages with palm trees and pools of water, and have the characters walk tantalizingly toward the island and jump into a pool, only to taste a mouthful of sand.

While mirages may not exactly be fanciful images of oases, people have reported seeing complex images like glaciers and simple ones like pools of water on highways. What is more, these images are neither the figments of a person’s imagination nor are they are the products of hallucination; in fact, mirages can be photographed and can be seen by anyone. The cause of these mysterious images is a relatively simple phenomenon known as “refraction.”

Refraction is a phenomenon in which light bends while traveling from one medium, such as air, to another, such as water. The simplest example of refraction is the famous “pencil in water” experiment, in which a pencil kept in a clear glass of water appears bent due to the bending of the rays of light coming out of the water into the air. Mirages are formed by a similar principle.
For mirages to form, the most important condition is that there have to be two distinct media for the light to pass through. Mirages are formed in regions where one part of the atmosphere is quite hot and hence not very dense, and another part of the atmosphere is relatively colder and much denser.

The junction of this cold and hot air thus provides an excellent place for the bending of light to occur. The next question that arises is how this simple bending of light causes viewers to see images of objects that don’t even exist.
The simplest example of a mirage is the pool of water that appears to be present in front of cars on highways on hot summer days. On such hot days, the roads are usually very hot and tend to the make the air right above them very hot as well. Higher above, however, the air is much cooler and also denser.

Thus, there are now two media: the hot air right above the road, and the colder air higher above. The light rays coming from the vehicles on the road that travel straight through the cooler air cause one to perceive the normal image of the car. However, it is the rays of light that travel downward toward the ground that make things interesting.
Rays traveling toward the ground enter the hot air above the road from the cooler air higher above. This change in media causes the light rays to bend upwards. Thus, instead of hitting the surface of the road, the rays of light enter a viewer’s eyes causing the viewer to see an inverted image of the car below the actual car. The brain perceives the inverted image of the car as a reflection of the car in a pool of water, and thus viewers see their “non-existent” pool of water on the highway.

Such mirages, formed due to the upward bending of light, are called “inferior mirages,” as the mirages normally appear below the actual object. The reverse of this phenomenon causes the formation of “superior mirages,” or inverted images above the actual object.
Perhaps the most intriguing example of a superior mirage is that of the sun. Light rays traveling from the sun to the earth travel from the hot air near the sun to the cooler air near the earth. This situation is the opposite of what happens in inferior mirages.
When the sun sets and sinks below the horizon, the bending of the light rays cause a mirage of the sun to appear above the actual sun. Thus, one perceives the sun to be well above the horizon when it has actually sunk. Another example of a superior mirage is the Arctic mirage described earlier.

More complex mirages, called “Fata Morgana,” are named after King Arthur’s shapeshifting half-sister. Fata Morgana are formed mostly in the polar regions, but have sometimes been formed in the deserts as well. These are formed when there are multiple layers of air of different densities. The effect of having such layers in the air is that distorted images of objects can be formed above, below, or even in front of the actual objects.

The images formed in a Fata Morgana may not necessarily be inverted but can be erect. An interesting effect of the Fata Morgana seems to be that many Arctic explorers in the early days recorded the existence of landmarks that never existed.
Distant objects projected their images in such a way that explorers claimed to see mountain ranges, peaks, or glaciers in places where there was actually nothing.