A change of direction that light undergoes when it enters a medium with a different density from the one through which it has been travelling. For example, when after moving through air, it passes through a glass slab or prism.

Refraction is the bending of a light ray when it enters from one medium to another medium. We know that the speed of light is different in different medium. It is fast in rare medium like air and it is slow in dense medium such as glass, water, etc.


So refraction is the bending of light ray when it passes from a fast medium (rarer medium) to a slow medium (denser medium). The amount of bending depends on the indices of refraction of the two media and is described quantitatively by Snell’s law.

Refraction is responsible for image formation by lenses and the eye.

As the speed of light is reduced in the slower medium, the wave length is shortened proportionately. The frequency is unchanged; it is a characteristic of the source of the light and unaffected by the medium change.



Snell’s law:

It is also known as the law of refraction.

Snell’s law is a formula used to describe the relationship between the angles of incidence and refraction; when referring to light or other waves passing through a boundary between two different isotropic media such as water, glass and air.


  • Like with reflection, refraction is also involves the angles that the incident ray and the refracted ray make with the normal to the surface at the point of refraction.
  • Unlike reflection, refraction also depends on the media through which the light rays are travelling. This dependence is made explicit in Snell’s law via refractive indices, numbers which are constant for given media.
  • Snell’s law: H


As in reflection, we measure the angle from the normal to the surface, at the point of contact.

The constant n is the indices of refraction for the corresponding media.

  1. The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lies in the same plane.
  2. The ratio of sine of angle of incidence to the sine of angle of refraction is constant, for the light of a given color and for the given pair of media.

If i is the angle of incidence and r is the angle of refraction then ratio of sin i by sin r is constant.

This constant value is called refractive index of the second medium with respect to the first.

Refractive index:

It is the ratio of velocity of light in vacuum and velocity of light in given medium such as glass, water, etc.

We also define it as the velocity of light in vacuum divided by the velocity of light in given medium.

Refractive index= Velocity of light in vacuum/ velocity of light in medium

n= c/v

  • Important point to remember:
  • Refractive index represented by a sign µ (meu). It has no unit because it is the ratio of two quantities.
  • The refractive index of water is 1.33.
  • The refractive index of glass is 1.66.

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