Light travels in a straight line, and we are well aware of that. This is true only on the condition that the light rays are traveling in the same medium, which has the same density all through. What happens if light travels from one transparent medium to another? Does it continue traveling in a straight-line path or change direction? A ray of light, passing through from water to the air, deviates (or gets refracted) away from the normal because it travels from the denser medium, which is water, into the rarer medium, air. A second ray of light refracts in some other direction. The two refracted rays when extended backward will converge at a point nearer the water surface than where it actually emerged. The image so formed is called a virtual image.

Thus, the submerged part of the stick seems to be raised and bent, and thus a virtual image is produced. It can thus be concluded that when the light rays are made to travel from one transparent medium to another transparent medium, then the light rays change direction at the boundary which divides the two media. Above is the example. If you see, in that situation, when the rays traveling inside the water enter another medium or air, then change its direction on entering into air. The change of light direction while passing obliquely from one transparent to another is called as refraction of light.

In other words, a change in the direction of light when passing obliquely from one transparent medium to another is called the refraction of light.

A transparent substance through which light travels. The medium in which light travels with greater speed is known as the optically rarer medium. Air, as compared to glass and water, is the optically rarer medium. A medium in which the speed of light is lesser is called an optically denser medium. Glass is more optically dense than water and air. Optical densities are assumed to vary in different media. The speed of light varies according to the optical density of the medium. The larger the difference in the speeds of light in the two mediums, the larger will be the deviation in the track of light in the secondary medium. In other words, there will be bigger refraction of light.

Laws of Refraction of Light

The refraction of light in travelling from one medium to another takes place according to two laws which are termed as the laws of refraction of light.

The First Law of Refraction: The normal, the incident ray, and refracted ray at the point of incidence, all lie in a similar plane.

The Second Law of Refraction: The second law of refraction provides a relationship amid the angle of refraction and the angle of incidence. This relationship was perceived experimentally by Willebrod Snell in 1621. Thus, the second law of refraction is termed as Snell's law of refraction.

According to Snell's law of refraction of light, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for the light of a given colour and for a given pair of media.

If i is the angle of incidence and r is the angle of refraction then according to Snell's law of refraction of light is given as,