Why are we able to see heat radiate off of things like cars and pavement.

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Why are we able to see heat radiate off of things like cars and pavement.

In: Physics

We cant see “heat”. What you see is hot air flowing upwards. In the lack of air, you won’t see anything.

It’s because of light [refracting](https://en.wikipedia.org/wiki/Refraction) through the heated air just above the surface of the road.

Refraction causes light (as well as other things like sound and waves) to bend when they move from one material to another. The heated air is different enough from the cooler air above it (due to density and heat expansion, etc) that the light refracts. It looks wavy like water because the way the light is being refracted changes as the boundary of the hot layer shifts due to wind and heating and stuff.

Hot air bends light similar to the way that looking through water bends light, you can’t actually “see” the heat itself, just the distortion it causes.

Hot air is less dense, and less dense air slows light down less than cold air does. Hot air has a lower index of refraction. Where there is a difference in refractive index, light bends and as the hot air currents from a car rise, that shows up as slight ripples in position of whatever light is passing through the cooler air into warmer air and back to cooler.

The mirage seen on roads in the distance is also due to a change in refractive index, but it is light from the sky which is totally reflected at the change in refractive index from warm air near the road to cooler air above. It only happens with shallow angles because the change in refractive index is not great.