The fake puddles of water that we see on the road on a sunny day is due to an optical phenomenon called a mirage, which is caused by the refraction (or bending) of light rays due to differing temperatures of the air above the road.
While driving on a hot, sunny day, you may have been surprised to see a puddle of water a few hundred meters ahead on the road. However, after covering that distance and reaching the spot where the puddle was, the water is nowhere to be seen. In fact, it has moved a few hundred meters further ahead on the road. Once again, you reach that spot, but find no trace of water.
How is it that an entire stretch of road can look drenched in water, and then be bone-dry by the time you reach it?
If you’ve ever chased these kinds of puddles of water on a hot day, you’ve already learned the hard way that there’s no water there. It’s actually an optical phenomenon, and acts as an illusion that confounds anyone and everyone that lays eyes on the road ahead on a steamy day.
How is light’s speed affected in a dense medium?
Light travels in a straight line with a constant velocity of 300,000,000 meters per second. However, its speed is affected by the medium through which it travels. Although light travels at this speed in a vacuum, it slows down a bit when it goes from a thinner medium (air) to a denser medium (glass, water).
This apparent slowdown occurs when photons (microscopic particles that make up ‘light’) interact with the particles of the medium they are traveling through. As a result, they are absorbed and emitted over and over again, which lowers the average velocity of light by a small fraction in a given medium.
The ratio by which light is slowed down in a specific medium is called the refractive index of that medium. It is a dimensionless quantity and its value is usually greater than 1 (although it can be less than 1 too).
How is the refraction of light related to ‘fake water’ on roads?
A medium’s refractive index partly depends on its temperature. Typically, the higher the temperature of the medium, the less dense it becomes, and vice versa. This effect is more noticeable in gases. The ‘fake water’ phenomenon is a very common example of a mirage, which is why it’s also sometimes called a ‘highway mirage’.
On a hot sunny day, the sun heats up the road a lot, just like everything else! However, since roads are generally black, they absorb a lot of heat and become even hotter than light-colored objects. This raises the temperature of the air just above the surface of the road. What you have there is a pocket of warm air below layers of ‘relatively’ cooler air. This creates a non-uniform medium, as the air just above the road becomes slightly less dense than the rest of the air.
Now, light rays from the sun travel through the air in a straight line, but when they reach the relatively warmer (and less dense) layer just above the road, their speed increases slightly and they change their path, getting refracted to reach the observer’s, i.e., your eyes.
Thus, the water that you see on the road is not really water, but a reflected image of the sky. Mirages are commonly observed on sunny days when the sweltering heat from the sun warms up flat surfaces (like roads) and thus the air above those sweltering stretches of asphalt.
Mirages can be pretty cruel, especially for thirsty travelers who are desperately looking for water in a sprawling desert. The optical illusion makes it seem as though there’s a water body nearby, because the water body (or at least an image) that appears due to a mirage is so perfect that you can’t really tell it apart from a real body of water. Parched travelers quickly head in that direction in the hopes of finding water, only to reach the point, find that there’s no water, just another mirage a few miles further along – a cruel trick of light, heat and optics!