Question

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 vaccum.
OR

What is meant by power of a lens ? Write its SI unit. A student uses a lens of focal length 40 cm and another of -20 cm. Write the power of each lens.

Answer 1

The two laws of refraction are :

(i) Incident ray, refracted ray and the normal at the point of incidence all lie in the same plane.
(ii) The ratio of sine of angle of incidence to the sine of angle of refraction at a point in a medium is constant.

Absolute refractive index : It is the ratio of speed of light in vacuum to the speed of light in the given medium.
Refractive index(n)= $\frac{\text{Speed of light in vacuum (c)}}{\text{Speed of light in medium (v)}}$

OR

The power of a lens is a measure of the degree of convergence or divergence of light rays falling on it. It is also defined as the reciprocal of its focal length in metres.

The S.I. unit of power is dioptre (D).
Let focal length of convex lens be F.
Focal length of a convex lens is positive.

So $F_{\text{Convex}}=40cm=\frac{40}{100}m$

Power =$\frac{1}{\text{Focal length}}=\frac{1}{\frac{40}{100}}$

=$\frac{100}{40}+2.5D$

So lens having focal length 40 cm is convex. Focal length of a concave lens is negative.

$F_{\text{Concave}}=-20cm=\frac{-20}{100}m$

So Power =$\frac{1}{\text{Focal length}}=\frac{1}{\frac{-20}{100}}$

= $\frac{-100}{20}=-5$ D

So lens having focal length -20 cm is concave.