Why is light bent around a hot surface?



For example, I look towards the roof of a black car on a sunny day. The background has enough contrast to notice that the light looks wavy from the radiated heat coming from the roof.

How does light get bent by the heat?

In: Physics

The surface heats up the air around it. Hot air is lighter, so it rises above, and it refracts air differently due to being less dense, causing these effects.

Hot air expands, and is less dense than cool air. And just like water and glass, air has its own certain refractive index, which changes with its density.

So whenever you look at an area where warm air mixes with cool air, you will see some distortions of the light passing through it.

You can also see this happen when some non-air vapor is mixing with air. At a gas/petrol station when fuelling a car, you can often see on the ground some weird shadows of the fumes rising from the nozzle. This is from the sunlight passing through that area being bent.

eta: This is also why we see mirages in the desert. When the air temperature gradient is just right, you can look at a spot below the horizon, where you *ought* to be seeing dry land, but instead you’ll be seeing bright blue light that’s been bent upwards from the sky. And your thirsty delirious brain tells you that this must be a reflection from a pool of water.

The hotter air gets, the less dense it is (which is why it rises. high density sinks, low density rises). When the density of the medium light is traveling through changes, it refracts a bit. Like when you go underwater in a swimming pool and look at something above the water, it’s not in the exact place you’re looking. It’s at a slight angle. It’s the same with light traveling through air that has a lot of different temperatures.

Why does light bend? Light goes through different mediums at different speeds. The light bends as it enters then new medium when it hits it at an angle. Imagine a car driving off the edge of a road onto the shoulder. You feel the car pull toward the shoulder because the tire is going slower in the gravel. Light which has the property of waves has one edge hit the glass (or warm air) at an angle and it bends at the boundary area.