The Science of Mirages

Desert Mirage in Cape Verde of the coast of Africa - By Ladislav Luppa via Wikimedia Commons

The Science of Mirages

We’ve all seen that part in the movie where the weary desert wanderer has been walking for hours and is dying of thirst. Then he happens upon a vast body of water on the horizon. He runs towards the water, it grows closer and closer, until he springs himself into the air only to land back down in the sand and no water in sight. You might think the traveler was hallucinating, but mirages are a naturally-occurring optical illusion. In cartoons, a mirage is often depicted as a peaceful, lush oasis lying in the shade of swaying palm trees, but in reality it is more likely to just look like a pool of water.

How do mirages form?

Mirages really have nothing to do with water at all. It’s really all about how light travels through air. Normally, light waves from the sun travel straight through the atmosphere to your eye. But, light travels at different speeds through hot air and cold air.

Mirages happen when the ground is very hot and the air is cool. The hot ground warms a layer of air just above the ground. When the light moves through the cold air and into the layer of hot air it is refracted (bent).

A layer of very warm air near the ground refracts the light from the sky nearly into a U-shaped bend. Our brain thinks the light has traveled in a straight line.

Our brain doesn’t see the image as bent light from the sky. Instead, our brain thinks the light must have come from something on the ground.

You can see from the image above what is going on in order for a mirage to appear. The “bent light from the sky” is refracted as it passes from cooler air into hotter air and back up to your eye. Our brains play a trick on us as it assumes that the refracted light follows a straight path. Because of this, we follow the light back to the source, which appears to be the ground. Combining all of this together, refracted light from the sky is interpreted as straight, letting us see an image of the sky on the ground.

This is why many mirages appear as blue water. We think we have stumbled on an oasis when in reality we are seeing a shimmering image of the blue sky. Since our brains don’t recognize the sky as being on the ground, we imagine the image to be shining blue water.

Where can you spot a mirage?

There’s no need to trek to the desert to see a mirage: they are very common on roadways, airport tarmacs and even on the hot sand at the beach. Mirages can be spotted anywhere where the ground can absorb a lot of heat.

The most spectacular mirages occur in wide expanses of flat land as too many hills, dips or bumps will prevent the refracted light from reaching your eyes.

Take a look at this video for more in depth information on the Science of Mirages and to see a mirage called a Fata Morgana where a city in China experienced seeing a floating city!


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