Chapter-9|Ray Optics and Optical Instrument|NCERT 12th Physics:


The bending of light when it passes from one medium to another of different optical density is called refraction.

When a beam of light encounters another transparent medium, a part of light gets reflected back into the first medium, while the rest enters the other. The direction of propagation of an obliquely incident ray of light that enters the other medium, changes at the interface of two media. This phenomena is called refraction of light.

Optical Density:

# Medium in which light travels faster is called optically rarer medium.

# Medium in which light travels slower is called optically denser medium.

* Optical density is measured by refractive index.

Refractive Index:

The refractive index(μ) of a material is the ratio of the speed of light(c) in vacuum to the speed of light in the medium(V).

Refractive index of medium μ= c/V

Note: 1. Optical density is entirely different from mass density.

2. Refractive index(μ) for air is 1.

Bending of Light:

If light travels from optical rarer medium to optical denser medium, then it bends toward the normal.
i.e.    i>r
If light travels from optical denser medium to optical rarer medium, then light bends away from the normal.

Note: 1.

2. If μ₁and μ₂ are refractive index for two medium, and v₁ and v₂ are speed of light in two different medium, then


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