Author: Sabrina Imbler / Source: Atlas Obscura
A desperate, thirsty traveler crawls among the dunes in a never-ending desert. Then something miraculous appears in the distance: the unmistakable blue of an oasis pool. You know what happens next. Of all the tropes about deserts—and there are a lot of them—the mirage may be the trope-iest.
But you don’t have to journey into the Sahara to see a mirage. In fact, the most spellbinding and storied example of this phenomenon happens in some of the coldest places on Earth and in the balmy archipelagos of the Mediterranean (probably the ideal place to kick back and get fooled by air and light).
Mirages are one of nature’s cruelest and coolest tricks. They’re not hallucinations, which are all in your mind, nor are they optical illusions, which are perceptual errors in vision in which you see something that isn’t real, according to Andrew T. Young, an astronomer and optical specialist at San Diego State University, in his online introduction to mirages. What you see when you see a mirage is a real, tangible optical phenomenon, even if your brain doesn’t necessarily know what it’s actually perceiving.
When you see light, your brain assumes it is coming to you in a straight line from the object emitting or reflecting it, says Jill Coleman, a professor of geology and atmospheric science at Ball State University. But when light passes through certain boundaries, such as where water meets air, or between air masses of different temperatures (and therefore densities), it gets bent, or refracted. And even when light is refracted, your brain still assumes it is coming to you in a straight line from its source, so refracted light holds the potential to scramble where things appear to be in your field of view. That’s a mirage.
There are two types of true mirages: inferior and superior, though those aren’t value judgments. Inferior mirages project the perceived image below the real position of the object. To conjure an inferior mirage, all you need is a flat, hot surface on a windless, sunny day—conditions that are met quite frequently in the desert, according to Coleman. The sun heats the flat surface, which creates a thin layer of warmer air directly above the ground. And when there’s no wind, it stays put. This refracts the light coming to you and seems to transpose a patch of sky and cloud onto the ground. It looks an awful lot like a patch of blue in the sand. That is how light and air trick our eyes into seeing what we most desire in the desert—a shimmering pool.
The mirage isn’t dependent on how hot it is, but rather on the difference in temperature between the air just above the ground and the air above that, Coleman says. Inferior mirages can happen just as easily on bright winter days, or on a slate roof, or above a barbecue grill—anywhere a sharp temperature change turns the air gauzy and rippling. These are everyday mirages, so common and familiar that we hardly notice them.
Then there are superior mirages, which occur when the temperature differential is flipped, and the air near the ground is colder than the air above. In this case the light bends up instead of down, and projects an image above the location of the real object. Superior mirages abound in colder climates, polar regions and oceans, Coleman says. Though much rarer in everyday life, superior mirages can result in an illusion so spectacular and…
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