Question

(a) State the laws of refraction of light. Give an expression to relate the absolute refractive index of a medium with speed of light in vacuum. (5)(b) The refractive indices of water and glass with respect to air are 4/3 and 3/2 respectively. If the speed of light in glass is 2 × 108 ms−1, find the speed of light in (i) air, (ii) water.

Answer 1

(a) Laws of refractionThere are two laws of refraction.First law of refractionThe ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. This is known as Snell’s law. Mathematically, it can be given as follows:nabHere, nab is the relative refractive index of medium a with respect to medium

b.Second law of refraction The incident ray, the refracted ray, and the normal to the interface of two media at the point of incidence − all lie in the same plane.If the light ray goes from medium 1 to 2 then the refractive index of medium 1 with respect to medium 2 is,where, v1 and v2 are the speeds of light in medium 1 and 2 respectively.(b) Given: Refractive index of water, nw = 4/3 and Refractive index of glass, ng = 3/2Speed of light in glass, vg = 2×108 m/s(i)(ii)

Answer 2

Explanation:

Snell's laws of refraction of light :

(1) : The incident ray, refracted ray and normal to the refracted surface lie in the same plane.

(2) : Ratio of the sine of the angle of incidence in medium 1 to that of the angle of refraction in medium 2 is equal to the ratio of the refractive index of medium 2 to that of 1.

$\frac{ \sin(i) }{ \sin(r) } = constant$

Absolute refractive index of the medium is defined as the refractive index of that medium with respect to refractive index of vacuum i.e. ratio of refractive index of the given medium to that of vacuum.

Absolute refractive index of medium =

$\frac{n \: medium}{n \: vaccum}$

=c/v

where v is the speed of light in that medium, c is the speed of light in vacuum.