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 further down the road. Once you have covered this distance and reached the place where the puddle was, the water is nowhere to be seen. In fact, it has moved a few hundred meters further down the road.
How can it be that an entire stretch of road looks soaked by water and then, when you reach it, is bone dry?
Anyone who has ever chased such puddles of water on a hot day has already learned the hard way that there is no water there. It is actually an optical phenomenon and acts like an illusion that confuses anyone who, on a steaming day, turns his eyes to the road in front of him.
How is the speed of light affected in a dense medium?
Light moves in a straight line at a constant speed of 300,000,000 meters per second. Its speed, however, is influenced by the medium it travels through. Light moves at this speed in a vacuum but slows somewhat when it goes from a thinner medium (air) to a denser medium (glass, water).
This apparent slowdown occurs when photons (microscopic particles that makeup ‘light’) interact with the particles of the medium they are traveling through. As a result, they are absorbed and emitted repeatedly, 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 common example of a mirage, which is why it’s also sometimes called a ‘highway mirage’.
On a hot, sunny day, the sun heats the road a lot, just like everything else! However, because roads are generally black, they absorb a lot of heat and become hotter than light-colored objects. This increases the air temperature just above the surface of the road.
What one has there is a bag of warm air under layers of relatively cooler air. This creates an uneven medium, as the air just above the road becomes somewhat less dense than the rest of the air.
Now the sun’s rays pass 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 course, being refracted to reach the eyes of the observer.
The water you see on the road is not really water but a reflection of the sky. Mirages are often observed on sunny days when the sweltering heat of the sun warms flat surfaces like roads, and thus the air above these muggy asphalt tracks.
Mirages can be pretty cruel, especially for thirsty travelers who desperately look 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.
Thirsty travelers quickly head in this direction, hoping to find water, only to find that there is no water, just another mirage a few kilometers away – a cruel trick of light, heat, and optics!