# NCERT Class X Science Class: Chapter –10. Light – Reflection and Refraction – Part-5

Question 20:

Identify the device used as a spherical mirror or lens in following cases, when the image formed is virtual and erect in each case.

(a) Object is placed between device and its focus, image formed is enlarged and behind it.

(b) Object is placed between the focus and device, image formed is enlarged and on the same side as that of the object.

(c) Object is placed between infinity and device, image formed is diminished and between focus and optical center on the same side as that of the object.

(d) Object is placed between infinity and device, image formed is diminished and between pole and focus, behind it.

(a) Concave mirror:

Object is placed between device and its focus, image formed is enlarged and behind it.

(b) Convex lens:

Object is placed between the focus and device, image formed is enlarged and on the same side as that of the object.

(c) Concave lens:

Object is placed between infinity and device, image formed is diminished and between focus and optical center on the same side as that of the object.

(d) Convex mirror:

Object is placed between infinity and device, image formed is diminished and between pole and focus, behind it.

Question 21:

Why does a light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself? Explain using a diagram.

When a ray of light is incident on a rectangular glass slab immersed in a medium, the emergent ray comes out parallel to the incident ray. The path of the light ray is as shown:

EF is the incident ray and GH is the emergent ray which is parallel to the incident ray.

Question 22:

A pencil when dipped in water in a glass tumbler appears to be bent at the interface of air and water. Will the pencil appear to be bent to the same extent, if instead of water we use liquids like, kerosene or turpentine? Support your answer with reason.

No. Bending will be different in different liquids since velocity of light at the interface separating two media depends on the relative refractive index of the medium.

Question 23:

How is the refractive index of a medium related to the speed of light? Obtain an expression for refractive index of a medium with respect to another in terms of speed of light in these two media?

The refractive index of the medium is given by

Let be the speed of light in medium be the speed of light in medium . The refractive index of medium with respect to medium is given by the ratio of the speed of light in medium and the speed of light in medium . This is usually represented by the symbol .

Question 24:

Refractive index of diamond with respect to glass is 1.6 and absolute refractive index of glass is 1.5. Find out the absolute refractive index of diamond.