Q22 of 60 Page 1

(a) State the laws of refraction of light. Explain the term absolute refractive index of a medium and write an expression to relate it with the speed of light in vacuum.

(b) The absolute refractive indices of two media 1A' and 1 B' are 2.0 and 1.5 respectively. If the speed of light in medium 1 B' is 2 x 108 m/s, calculate the speed of light in: (i) vacuum, (ii) medium 1A'.

(a) The laws of refraction of light-

1. The incident ray, the refracted ray and the normal at the point of incidence, all lie in the same plane.


2. The second law of refraction of light is the Snell's Law of Refraction. It states that the ratio of sine of the angle of incidence to the sine of angle of refraction is a constant for a given pair of medium.


=AμB


This AμB is called refractive index of the medium B with respect to medium A.


The ratio of speed of light in vacuum to the speed of light in a medium is called the absolute refractive index of that medium,


μB=


Where c = speed of light in vacuum


v= speed of light in medium B


Here, μB is the absolute refractive index of medium B.


(b) μA= 2.0;


μB = 1.5;


vB = 2 x 108 m/s


(i) Speed of light in vacuum, c = ?


μB= 1.5 = c/vB


c = μBvB = 1.5 x 2 x 108 m/s = 3 x 108 m/s


(ii) For medium ‘A’ : μA= c/vA



Thus the speed of light in medium A is 1.5x 108 m/s


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