What causes a mirage in the road?

30 May 2010

Question

When driving around at this time of year and looking ahead, you see what looks like a reflection of water on the road. I know this is a mirage, but what is it – what are we actually seeing and what are the conditions that are making it happen?

Answer

Dominic - This is interesting because of course, we normally think that light travels straight lines and so, when light comes into your eye, you know what direction it's come from, so you know what direction you're looking in. But light doesn't always travel in straight lines. For example when it goes through a lens, we know it's bent and that's because the lens is made of a glass which has different refractive index from the air around it. Air also has its own refractive index which depends on the temperature of the air. So, if you have hot air, it has different refractive index to if you have cold air. On a hot day, the sun will come down, it will heat the surface of the road and make the air close to the road very hot in comparison to the air above it. That means that light rays are bent away from the road, and so, when you look down at the road, the rays are actually bending away from the road and back up into the sky, and you're seeing a patch of the sky in the road. Water also looks like that because water is reflecting the sky, making the sky appear in the road. So the mirage looks exactly like water.

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