Why do roads look reflective?

11 November 2007

Question

While driving on a long stretch of highway I notice that when one looks ahead, very far down the road and on curves (left, right, up and down) the surface of the road becomes almost reflective. I’ve noticed this under both sunny and cloudy conditions but I’m wondering why it is.

Answer

Dave: This is called a mirage. On a hot day, the air above the road gets hotter than the air above. When air gets hot it expands and becomes less dense. The less dense the air is, the faster light goes through it. So light is going slower high up than it is close to the road. This means that light refracts as is comes down from the bright sky; this bends it upwards and into your eye. So you end up seeing is an image of the sky in front of you. This makes the road look reflective and appear to shimmer, almost like there is a sheet of water on the surface - but that is the blue light of the sky, not water!

Chris: When you have toast cooking in your toaster and you look at the air above it, it's also all twisty and shimmery...

Dave: It's the same phenomenon. You get hot air in swirly patterns above the toaster that bends the light which produces a distorted image behind.

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