Mirage: why do we see a pool of water?
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Walking through a baking desert, you see a distant pool of water towards the horizon. An oasis to save the day, perhaps? No—just a mirage. But what causes it, and why does it look like a pool of water?
I remember the satisfying Aha! moment I experienced when I learned that the apparent pool of water is simply a reflection of the sky above appearing on the ground. Because it looks like a reflection of whatever’s above, our brains go for the most likely explanation: it must be water.
What causes a mirage?
A few things come together to trick us.
First, on a hot day in a hot place—such as a tarmac road in a desert—the ground heats up strongly. This warms the air just above it, creating a temperature gradient: the air nearest the ground is much hotter than the air above.
Second, light refracts, or bends, as it passes through mediums of different densities. When you see a bent straw in a glass of water. Light travels at different speeds in air and water, and the change in speed makes it change direction.
On our hot road with a temperature gradient in the air above, the hotter air near the surface is less dense than the cooler air above. As light passes through these layers, it bends smoothly. Under the right conditions, light from the sky may bend so much that it curves back upward before reaching the ground, eventually entering your eyes.
Lastly, when we see sky-coloured light coming from the ground, our brains interpret it as a reflection—so we perceive a shimmering pool of water.
Perhaps it's easier with a picture...
Types of mirages: inferior and superior
Strictly speaking, a mirage like this one—the most common to experience—is an inferior mirage, where the image appears below the real object. For example, a car driving on a hot road can look as though it’s reflected in a pool beneath it.
There's also a rarer but striking superior mirage, where the mirage appears above the real object. This happens with the opposite temperature gradient—when colder air lies below warmer air—such as in winter or over polar ice. The results can be startling, such as ships that seem to hover above the sea .
