Why Do We See Mirages?
For the people who are reading the word ‘mirages’ for the first time, a mirage is that illusion of mirror you see on a hot summer road. We are going to explore the science behind this mirage and find out how a human brain perceives it.
These type of mirages are the most common mirages we experience. The most common place you can experience it is the asphalt highway, which is scorched by our star on a superhot summer day. The asphalt usually has a layer of hot air as the heat absorbed by it is transferred via convection. This creates a thermal gradient over the asphalt with hot air just above it and warmer/colder air above the hot air. Therefore, when you look at the road from a certain distance, you could see the wavy mirage with a bluish layer acting like a mirror.
The reason behind this phenomenon is due to the behavior of the photons in different medium. Photons usually travel in a straight line inside a homogenous medium. In our case of hot road, the medium in which light travels is air. If the temperature and density of the air is uniform, the light travels in a straight line. However, in our hot road, we have a temperature gradient with hot air and warm air above it and so on. Which means, we have varying temperature and density. Therefore, when light travels from the sky it bends (refracts) through this different medium. The reason why it bends is that it’s the property of photons to bend in a less dense medium (hot air), as the refractive index is low compared to the refractive index of a denser medium (cold/warm air). This is explained by QED too. Photons always prefer a faster path when it travels. In our hot road, the faster path is the hot air.
When you are viewing at the road, initially, the light waves of the vehicles travel in a straight line to your eyes. Besides that, a few other part of the light waves take a faster path by bending through a medium with lower refractive index and reach your eyes before the initial light wave. This tricks your brain and you perceive it as a light wave that is travelling in a straight line. This creates the illusion of a giant wavy mirror on the road, as what you see will be a blue sky, which is mostly interpreted as water or mirror. The waviness is because the air is not always stable, as the hot air tend to rise due to the fact that it is less dense than the air above it.
Superior mirages can be found on an ice sheet or colder regions, where the gradient is reversed. Ice sheets usually have colder film of air just above it. Above that film of cold air, will be the normal hot air. This gradient cause the reverse effect when seen from a certain distance. The object you are looking at will seem to be reflected above it or the horizon. A rare example of this can be seen on the beach, when you are looking at a far distant ship. When the conditions are favorable for the superior mirage, the ship would appear again above the surface.
When mirages appear the next time on the road, you now have an awesome physics story to tell your family and friends. If you are interested further, you can even demonstrate the refraction with a glass of water and some arrows drawn on a paper behind it. At the end, you can even demonstrate a small experiment explaining Snell’s law.
This post was first published on October 14, 2015.