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What is a mirage?

Under a baking sun, a weary traveller trudges across a seemingly never-ending expanse of desert. Looking up, he suddenly spots something in the distance: a sparkling lake. He rubs his eyes. It’s still there. Picking up the pace in glee he strides ahead… only for the water to melt into thin air.

You might think our traveller 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 much more likely to look like a pool of water.

The illusion results from the way in which light is refracted (bent) through air at different temperatures. Cold air is denser than warm air, and therefore has a greater refractive index. This means that as light passes down from cool to hot air, it gets bent upwards towards the denser air and away from the ground (see diagram).

To your eyes, these distorted rays seem to be coming from the ground, so you perceive a refracted image of the sky on the ground. This looks just like a reflection on the surface of a pool of water, which can easily cause confusion.

Diagram of a mirage. Credit: Anton

A beginner’s guide to mirage spotting

1. There’s no need to trek to the desert to see a mirage: they are very common on roads. In fact, likely locations include anywhere where the ground can absorb a lot of heat. If you’ve ever walked barefoot on hot tarmac or sand, then you’ll know just how hot they can get! A hot ground warms up the air immediately above it, creating a sharp temperature gradient in the air – the first ingredient of a good mirage.

2. Make sure you can see ahead of you well into the distance. 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.

3. Check the weather forecast. If you see a huge puddle ahead on a rainy day you’d be better off steering well clear, as mirages are far more likely to found during dry, sunny weather.

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