Why does road look Wet in Hot Summer Days?

You might have experienced something which almost every driver experiences, seeing a puddle down the road on a hot summer day, but have never been able to reach it. When you look further down the road you can see another puddle but still, cannot reach it. You can choose to chase these puddles all day, but you won’t ever actually reach one. This is caused by the same reason that the cartoon characters, which are lost in the desert, think they see a mirage which is usually caused by refraction of light.[irp]

I would like to start with a short lesson in physics. Light moves at a slower rate through dense mediums and faster through less dense mediums. As the light travels from one medium to a certain density to another of a different density, the light is forced to change speed, which causes it to change direction. This is known as refraction. An easy experiment to see the refraction process occur would be to put a straw in half a glass of water. If you look at the straw from the top, it looks like the straw is bent or crooked. From the side, it looks like it is getting wider as it enters the water. The light refracts not only through different mediums, it also refracts when moving through layers of the same medium with different densities. As the light from the sun travels down towards the hot roads, it moves through increasingly hot and less dense layers of air, which causes a change in speed and refracts as it moves through them. It sorts of takes a U-Turn path towards the road and then back up, which is usually caught by the eye of anyone standing farther down the road.

When the refracted light eventually reaches you, your eye and brain like the straw in the water- doesn’t account for the bending it did along the way from the sky. The brain traces it back to a straight line which interprets the point as the location and origin of the object. Essentially what you see is a part of the sky sitting on the road. Even while your brain and eyes are trying to make sense out of what you are viewing, the brain knows that the sky on the ground makes no sense; hence you perceive this as a mirage of water on the road.

Turbulence or change in wind patterns can cause this mirage to get distorted, which in turn strengthens the effect. Sand, which is found near almost all highways in the Kingdom, is a good conductor of heat and holds onto it for a long time. This causes the air around it to also heat up, which eventually causes these mirages of water that are a common occurrence in the desert. This can fool travelers into thinking that there is water nearby.[irp]

This can be particularly dangerous for those who become stranded in the desert, as they keep going towards the mirage for water and shade; however, they get lost in the desert as they do not keep track of the direction and distance that they have walked.


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