— Why does heat distort what I see?
In: Physics
Light doesn’t travel in a straight line all the time. Among other things, when it moves between different materials, like air into water, it changes direction depending on the materials. This is a whole day of physics class, and part of why a prism makes a rainbow.
Well hot and cold air count as different here, and light curves when it hits a pocket of particularly hot or cold air. This causes the distortions, from roads that look wet at a distance but aren’t and other mirages, to just plain hot air looking like it’s some kind of cloud.
Hot air is probably at a different density from its source and light bends when it passes through denser material like water.
You know how water refracts light differently from air? That’s why a straw in a glass of water looks like it is bent.
Well, air also changes refraction depending on temperature. If it’s just generally very hot or very cold everywhere you won’t notice a difference, but if there are temperature differences, hot air will rise up. Along the way the hot and cold air will swirl around a bit and that means you have areas with different refraction swirling around.
It’s most noticeable when you have some heat source at the bottom (like hot asphalt) that heats the air at the bottom and makes it rise up.
It’s called refraction and is actually the same effect as when you look at a straw through a glass of water and it appears bent. As the light passes through a denser medium (water), it changes speed and direction.
In the case of the “waves” you see coming off a hot object, the hot air close to the object is less dense than the air directly above it and therefore light is refracted as it slows down in the cooler air. The heat is also being transfered from the hot air to the cold while you’re looking at it so it’s not constant or stable like air/water refraction, but it is the exact same effect.
When light crosses between two materials it gets bent. This is how prisms work, or maybe you’re familiar with [the light deflection between water and air?](https://media.sciencephoto.com/c0/30/81/62/c0308162-800px-wm.jpg)
Anyway, air’s density changes a lot with temperature. Hot air and cold air have very different densities and therefore bend light differently. In fact they’re so different that it’s like hot air and cooler air are two different materials, so you end up seeing light bending and distorting as it crosses temperature-changes in air, just like what happens when light goes from air to water etc.
Add to that that the hot air above direct heat sources like open flames, BBQs, and hot asphalt tends to get all swirly as hot air rises and causes turbulence. So you don’t just have one hot-cold air boundary, you have hundreds of hot-cold-hot gradients and they’re all moving and changing, each one of them bending the light a bit as it passes through. So overall it makes the whole area look all shimmery as the light gets bent and re-bent by all the temperature changes on the way through.
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