Why do roads look glossy or wet on a hot day but disappear when you get closer to it?
In: Chemistry
When air gets heated, it expands. When light shines through air of different temperatures, it bends (just like a straw in a glass of water).
On a hot day, the dark tar of the road heats up the air directly above the road surface. This means the air is hotter and less dense right above the road, but gets cooler and denser as you go up. A ray of light shining down at the road can get gradually bent by the changing density of air. As it gets closer to the road, it bends more and more until it completely reverses direction and starts to shine slightly upwards. When you see the shimmer, you’re actually seeing the sky, almost as a reflection off the road. I say almost because the light never actually reaches the road.
As you approach this mirage, the angle gets too steep and you see through to the road below.
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