While traveling on a hot day, have you ever noticed wavy lines on the road lying ahead? Or even, over a hot grill? What causes those wavy lines to appear?
Actually, they are the effect of heat on light waves. The wavy lines are brought about by a phenomenon called light refraction.
What is light refraction?
The refraction of light occurs when the light waves bend when light passes between two different media. This is measured in terms of refractive indices. A refractive index is the measure of the bending of a ray of light when it passes from on medium to another. A specific medium has a refractive index of its own. Therefore, when light passes from one medium to another, it bends accordingly.
The basis of a Mirage
Coming back to the subject at hand of the wavy lines seen during periods of intense heat. This occurs as a result of light moving from an area of hot air to an area of cold air.
So, basically, hot air and cold air have different refractive indices. Hot air is less dense than cool air: this causes light to speed up through the hot surface and curves back upwards, such that an image of the sky and the surface appears to be wavy. Reminds you of mirages in the desert?
That is exactly how mirages are formed. The extreme heat in desert environments causes viewers to see illusionary images from a distance. Oasis-mirages are only the result of light waves bending differently.
We view the effects of refraction of light as if they were just a reflection of the image being seen. In fact, refraction and reflection are quite similar, as captured by the human eyes: this is how we often mistake effects of light refraction to be actual objects. Ever noticed pools of water on the road when there is no such thing on the road, and you only realize this when you get closer? Again, the refracted image of the sky from the extremely hot road appears to be the reflection of the sky in pools of water.
Now, what causes the waviness of the images? The objects appear to be wavy as opposed to straight and stable because the air temperature does not remain fixed at one point. As parts of the air get hotter than others, they rise since they become less dense. When this air cools again, it sinks to the bottom and gets heated again, as if creating a cycle. This movement of air molecules brings about vibrations such that the images that result thereof appear to be wavy.