Refraction of light waves

REFRACTION OF LIGHT WAVES

Refraction is the phenomenon in which light travels from one medium (say air) to another medium (say glass). The direction of light changes due to change in medium.

If a ray of light passes from a rarer medium (air) into denser medium (glass) then the ray of light bends towards the normal. (Fig 4.2)

If a ray of light passes from denser medium into rarer medium then the ray of light bends away from the normal. (Fig 4.3)

Note: The speed of light in rarer medium is faster than that of denser medium

Laws of refraction

(i) The incident ray, the refracted ray and the normal at a point of separation of two media lie in the same plane.

(ii) For any two medium, the ratio of sine of angle of incidence to sine of angle of refraction is constant. It is known as Snell's law.

where this constant is called as refractive index (μ) of medium

Significance of refractive index

The ratio of velocity of light in vacuum to velocity of light in medium, is called as refractive index.

(i) When light travels from rarer medium (air) to denser medium (glass), then the ∠i is greater than ∠r. Therefore, refractive index is always greater than 1. 

(ii) When light travels from denser medium (glass) to rarer medium (air), then the ∠i is less than ∠r. Therefore, refractive index is always less than 1. 

(iii) Refractive index for vacuum is unity (1)