Why do roads look glossy or wet on a hot day but disappear when you get closer to it?
In: Chemistry
I learnt this earlier this year in physics so I don’t really remember but it has something to do whit reflection and refraction (light phenomenons) of the sunlight.
The heat from the road creates a layer of hot, less-dense air above it. Light travelling from the cooler air above through the warmer air refracts, bending light from the horizon or sky towards you, thus why it has a lighter color.
The glossy effect is because the hot air is less dense and rising, being replaced by cooler air which is then heated by the road, creating a moving cycle which alters the exact amount of refraction occurring and thus making it look all shimmery.
The refraction only bends light so much though and thus usually requires a low viewing angle for light from the sky to be seen. As you get closer, the viewing angle increases, so even with the refraction, the light hitting your eyes is coming from the road instead of the sky, thus it no longer has the bright, watery look.
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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