Why does the road look like it's WET on a hot day? The Physics behind a Mirage

Have you ever been on a road trip in the heart of the summer? You may have noticed that in the distance you see there is a puddle on the road. As you drive closer to it you realise there is not even a drip of water. This is called a Mirage. What is the Physics behind it and how does it work?

We must first understand what refractive index is.

Every object has a refractive index. It is simply a measure of how light travels through a medium, for example glass. The higher the refractive index, the denser the medium so the slower light travels through it. You can prove this with a very simple experiment at home!

Fill up a see through cup with water and put a pencil in it. You will notice that the pencil seems to be sliced in half from the point of contact of the water. This is because the refractive index of the water is higher than the refractive index of air. This means light travels slower through the water hence why the pencil is refracted.

Figure 1: An image of what to expect from the experiment of pencil in cup of water

How does this relate to a Mirage?

Mirages tend to happen when the weather is hot. As the heat from the sun heats up the ground, the ground heats up a thin layer of air just above it. This now creates 2 separate mediums; thin hot layer of air just above the ground, and colder air above this warmed up layer. This creates a refractive index gradient as light travels through hot air and cold air at different speeds.

So... as the light from the sun in the cold air travels into the thin layer of hot air, refraction occurs. Effectively the light is bent back out forming a "U" shape, creating a virtual image. The picture below explains this very well. Please note that the apparent image which is labelled on the picture IS the virtual image.

Figure 2: An image showing how a Mirage occurs

In conclusion - due to refraction (because of a gradient in the refractive index of cold and hot air), the image we see is virtual and is of the sky as if there is a mirror laid on the floor. If there was water on the floor, it would reflect light and also show a virtual image of the sky hence the appearance that the floor is wet. This is the physics behind Mirages!

Any questions leave them below otherwise, until next time, take care.

~ Mystifact