what is that weird “mirror” like effect on top of asphalt when it gets hot? What’s the science behind it?


what is that weird “mirror” like effect on top of asphalt when it gets hot? What’s the science behind it?

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The separation of visible light into its different colors is known as dispersion. Each color is characteristic of a distinct wave frequency; and different frequencies of light waves will bend varying amounts upon passage through a prism…or in this case, oil.

Because the asphalt is black is absorbs way more heat than the air. This heat is still being let off and heating the air just above it. That mirror like effect is your vision looking through heated air.

Imagine the air you see when you look above a heat source the swirly lines that ripple in the air

On a sunny day there will be a layer of hot air sitting on top of the asphalt, because asphalt is very good at absorbing sunlight. Being hot, this air is lower density than the less heated air above it, and whenever you get two transparent areas with different density, you get refraction, or light bending. You can easily see this by filling a bowl with water and sticking a pencil in at an angle–the pencil will appear to bend at the point it enters the water, because water is denser than air. You get the same effect on the asphalt, which causes various odd effects including mirroring, shimmers, or even being able to see things you wouldn’t normally be able to see because they’re over the horizon (that’s called a mirage).

Rising hot air can refract (bend) the light. The moving air is what causes the ripples in the image. hot air is less dense than cold air so photos can travel (very so slightly) faster.

The “mirror” effect is called a mirage. Light from a distant object that is moving towards the hot ground halfway between you and the object will bend up as it moves through the less dense hot air. When that light strikes your eyes you see an inverted image on the ground in between you and the object. Unlike a mirror image, this image shifts because hot air rises and the bending angles aren’t quite constant. In nature we also see shifting images on the ground when there is water on the ground. Our brain interprets the shifting image as “the ground over there must be wet”. This can lead people to chase non existent distant water when lost in the dessert.

It’s called a “Schlieren Effect”. When light moves from one transparent material to another, it can bend. You see that with a prism, or when looking into a pool of water. It’s because there’s a difference in how quickly light can move through different materials.

It turns out, when you take a single transparent material, but vary how dense it is, the same thing happens – the Schlieren Effect.

In the case of the road, the asphalt absorbs a lot of sunlight and gets very very hot. The air next to the asphalt then gets really hot, and when air gets hot, it expands and is less dense than the surrounding air and the light passing through it bends enough that you see the light of the horizon appear where the road is.