Refractive Index

Ever wondered what causes a rainbow to appear in the sky? Several phenomena together give rise to the formation of a rainbow. Refraction is one among the several phenomena responsible for the beautiful display of rainbow in the sky. Refraction is referred to the process of bending of light wave path as light passes from one medium to another medium. Here, we will study what a refractive index of a medium is and how it affects refraction.

What Is A Refractive Index?

The speed of light in a medium depends on the properties of the medium. In electromagnetic waves, the speed is dependent on the optical density of the medium. Optical density is the tendency of the atoms in a material to restore the absorbed electromagnetic energy. The more optically dense a material is, the slower the speed of light. One such indicator of the optical density of a medium is the refractive index. Before we look at the relationship between the two, let us first understand what refractive index is.

The Formula Of Refractive Index

Refractive index is dimensionless. It is a number that indicates the number of times slower that a light wave would be in a material than it is in a vacuum. Refractive index, represented by symbol n, is the velocity of light in vacuum divided by the velocity of light in a medium. The formula of refractive index is given as follows:

n=c/v (1)


n is the refractive index

c is the velocity of light in vacuum ( 3 × 108 m/s)

v is the velocity of light in a substance

The vacuum has a refractive index of 1. The refractive index of other materials can be calculated using equation (1). Higher the refractive index, higher the optical density and slower the speed of light. The table below lists the refractive index of different media.


Refractive Index









Ethyl Alcohol


Example of refractive index

The refractive index of glass ng is 1.52 and that of water nw is 1.33. Since the refractive index of glass is higher than the water, the speed of light in water is faster than the speed of light in glass. If the refractive index of a medium is greater than that of another, then the first medium is said to be optically denser. Most of the substances we know have a positive refractive index having value more than zero. Substances with negative permittivity and negative permeability have a refractive index of negative value.

The refractive index provides a measure of the relative speed of light in different media. Knowing the refractive indices of different media helps the student to identify the direction in which way the light would bend while passing from one medium to another.

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Practise This Question

Figure below shows a small wheel fixed coaxially on a bigger one of double the radius. The system rotates about the common axis. The strings supporting A and B do not slip on the wheels. If 'x' and 'y' be the distances travelled by A and B in the same time interval, then