Ray Optics – Notes – 2


The bending of the ray of light passing from one medium to the other medium is called refraction. 

Laws of Refraction

The refraction at a plane surface always takes place in accordance with the following two laws :
(i) The incident ray, the reflected ray and normal to surface separating two medium all lie in the same plane.


Figure-1 : Refraction

(ii) The ratio of sine of the angle of incidence to the sine of the angle angle of refraction (r) is a constant called refractive index.


For two media, Snell’s law can be written as :


Refractive Index :

The ratio between the speed c of light in vacuum to the speed v in a given material is called the index of refraction μ of that material :


Refractive index of a medium is that characteristic which decides speed of light in it. It is a scalar, unit less and dimensionless quantity.

Dependence of Refractive index :

(i) Nature of the media of incidence and refraction.
(ii) Colour of light or wavelength of light.
(iii) Temperature of the media : Refractive index decreases with the increase in temperature.

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