Why does the road surface appear to shimmer when it’s hot?

01 June 2008

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Question

Why does the road surface appear to shimmer when it’s hot?

Answer

When air is hot it expands and light will go through hot air slightly faster than cold air. When light changes its speed at an angle it will bend like when it goes into glass or something. You get lots of turbulence with air moving up which light will go through faster. So you get light bending in all sorts of directions which change as the air moves and you get this turbulence effect. The same effect makes stars twinkle, as you get turbulence up in the atmosphere which bends the light coming from stars which makes them twinkle on and off. Sometimes they come towards you, sometimes they go the other way.

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