Question

A concave lens made of a material or refractive index n1 is kept in a medium of refractive index n2 A parallel beam of light is incident on the lens. Trace the path of light parallel to principal axis incident on concave lens after refraction when:
1) ${n_1} > {n_2}$
2) ${n_1} = {n_2}$

Answer 1

Hint:Refractive index: The refractive index of a material is a dimensionless number that describes how fast light travels through the material. It is defined as
$n = \dfrac{c}{v}$
Where c is the speed of the light in vacuum and v is the phase velocity of the light in the medium.
Law of refraction: the law of refraction predicts that a light-ray always deviates more towards the normal in the optically denser medium: i.e., the medium with the higher refractive index.

Step by step solution:
1) ${n_1} > {n_2}$

In this case medium of concave lens is denser than the medium from where light ray is incident on it. Therefore, at entering the surface, the light ray bends towards the normal ${n_1}$ , while leaving the lens, the ray again goes under refraction at leaving surface and in this situation it bends away from the normal ${n_2}$ .

2) ${n_1} = {n_2}$

In this case light ray parallel to the principal axis goes straight without any deviation as the refractive index of both the medium are same hence no change in the speed of the light while transitioning from one medium to another.

Note:many times we get confused between the term refraction and reflection. Both are the terms of optics but very different phenomena hence we should have a clear understanding of both concepts.