Diamond has a refractive index of 2.42 which means that the speed of light in diamond will reduce by a factor of 2.42 as compared to its speed in the air.

In other words, the speed of light in diamond is 1/2.42 times the speed of light in vacuum.

Question - 12 : - Define 1 dioptre of power of a lens.

Answer - 12 : - Dioptre is the SI unit of power of lens is denoted by the letter D. 1 dioptre can be defined as the power of a lens of focal length 1 metre.

Question - 13 : - A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it.

Answer - 13 : - Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also, find the power of the lens.

Answer

The position of the image should be at 2F since the image is the real and same size.

It is given that the image of the needle is formed at a distance of 50 cm from the convex lens. Therefore, the needle is placed in front of the lens at a distance of 50 cm.

Object distance (u) = – 50 cm

Image distance, (v) = 50 cm

Focal length = f

According to the lens formula,

Question - 14 : - Find the power of a concave lens of focal length 2 m.

Answer - 14 : -

Focal length of concave lens (f) = 2 m

Power of lens (P) = 1/f = 1/ (-2) = -0.5D

Question - 15 : - Which one of the following materials cannot be used to make a lens?

Answer - 15 : -

(a) Water (b) Glass

(c) Plastic (d) Clay

Answer –

(d) Clay cannot be used to make a lens because if the lens is made up of clay the light rays cannot pass through it

Question - 16 : - The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object?

Answer - 16 : -

(a) Between the principal focus and the centre of curvature

(b) At the centre of curvature

(c) Beyond the centre of curvature

(d) Between the pole of the mirror and its principal focus.

Answer-

(d) The position of the object should be between the pole of the mirror and its principal focus.

Question - 17 : - Where should an object be placed in front of a convex lens to get a real image of the size of the object?

Answer - 17 : -

(a) At the principal focus of the lens

(b) At twice the focal length

(c) At infinity

(d) Between the optical centre of the lens and its principal focus.

Answer –

(b) The object should be placed at twice the focal length

Question - 18 : - A spherical mirror and a thin spherical lens have a focal length of -15 cm. The mirror and the lens are likely to be

Answer - 18 : -

(a) both concave

(b) both convex

(c) the mirror is concave and the lens is convex

(d) the mirror is convex, but the lens is concave

Answer –

(a) Both are likely to be concave.

Question - 19 : - No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be

Answer - 19 : -

(a) plane

(b) concave

(c) convex

(d) either plane or convex

Answer –

(d) The mirrors are likely to be either plane or convex

Question - 20 : - Which of the following lenses would you prefer to use while reading small letters found in a dictionary?

Answer - 20 : -

(a) A convex lens of focal length 50 cm

(b) A concave lens of focal length 50 cm

(c) A convex lens of focal length 5 cm

(d) A concave lens of focal length 5 cm

Answer –

Free - Previous Years Question Papers

Chapter 10 Light Reflection and Refraction Solutions

Subscribe Now!