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

In Physics by Mystifact

Have you ever been on a road trip on a hot, sunny day? You may have noticed that in the distance there seems to be a puddle on the road. As you drive closer towards it, you realise there is not even a drip of water. This is called a Mirage. What is the Physics behind one and how do they give off this illusion?

The Physics behind a Mirage

To begin to understand a Mirage we must understand what refractive index is.

What is Refractive Index?

Every object has a refractive index. It is simply a measure of how fast light travels through a medium. Let’s take glass as an example. 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.

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, a warm, thin layer of air forms just above it. This now creates 2 separate mediums; a 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. In fact, it occurs so much that light enters and leaves at such a steep angle (effectively, the path of light looks like a “U”). The picture below explains this very well. Please note that the apparent image which is labelled on the picture is the reflection that you see which makes it seem like there is water on the ground!


In conclusion, due to refraction, the image we see is virtual as if there is a mirror laid on the floor. If there was water on the floor, it would reflect as opposed to refract which would also show a virtual image (as if there was a mirror on the floor). This is why it seems like there is water in the distance on a hot day. This is the physics behind Mirages!

If you have any questions, leave them below and until next time, take care.

~ Mystifact

Please note; no copyright infringement is intended. All images used have been labelled for re-use on Google Images. If any artist or designer has any issues with any of the content used in this article, please don’t hesitate to contact me to correct the issue.

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