What is a Mirage?

Ever wondered, why you see distant cars' distorted mirror images on the tar road on the hot sunny day? Well, these images are called Mirage. It is naturally occurring optical phenomena in which light rays are bent and produce displaced image of distant objects. The English word "Mirage" is having root in Latin word "Mirare", which means "to seem". Mirage is not an optical illusion. One can take photographs of it.

Unlike mirror, the principal physical cause for a mirage is refraction rather than reflection. Refraction is the change in direction of a wave due to a change in its speed. It is measured by Refractive Index. Refractive Index is the measure for how much the speed of light (or other waves such as sound waves) is reduced inside the medium. For example, typical glass has a refractive index of 1.5, which means that light travels at 1 / 1.5 = 0.67 times the speed in air or vacuum.

Cold air is denser than warm air, and has therefore a greater refractive index. That means, light waves become slower in cold air compared to hot air. As light passes from colder air to warmer air it bends away from the direction of the temperature gradient when it passes from hotter to colder, it bends towards the direction of the gradient.

The diagram above shows a light ray coming from the sky toward the hot ground. If the air near the ground is warmer than that higher up, the light ray bends in a concave upwards trajectory. Once the ray reaches the viewer’s eye, the eye traces it as the line of sight, which is the line tangent to the path the ray takes at the point it reaches the eye. The result is that an inferior image for the above sky appears on the ground.