PYQs (Ch 9: Light – Reflection and Refraction)

These are some PYQs (previous year questions) from the CBSE board for Class 10 Science, Chapter 9: Light – Reflection and Refraction.

By practicing these questions, you will get an idea of what type of questions may appear in the examination.

Question 1:
In which of the following is concave mirror used?
(a) Solar cooker
(b) Rear view mirror in vehicles
(c) Safety mirrors in shopping malls
(d) In viewing full size image of distant tall buildings

1 mark; CBSE 2022

(a) Solar cooker. Because concave mirrors are converging mirrors and they reflect sunlight at a single focal point.

Question 2:
An optical device forms an erect image of an object placed in front of it. If the size of image is one half of the object, the optical device is a
(a) concave mirror
(b) convex mirror
(c) plane mirror
(d) convex lens

1 mark; CBSE 2022

(b) convex mirror. Because the image formed by concave mirror is always erect and of a smaller size.

Question 3:
The relation R = 2f is valid
(a) for concave mirrors but not for convex mirrors
(b) for convex mirrors but not for concave mirrors
(c) neither for concave mirrors nor for convex mirror
(d) for both concave and convex mirrors

1 mark; CBSE 2022

(d) for both concave and convex mirrors

Question 4:
If the image formed by a convex lens is of the same size as that of the object. What is the position and nature of the image with respect to the lens?

2 mark; CBSE 2013, 2012

The image will be real, inverted and will be formed at centre of curvature of the lens.

Question 5:
How can you identify the three types of mirrors without touching them?

2 mark; CBSE 2011

a. If the image formed is erect, of same size, and at equal distance behind the mirror as object and is in front of the mirror, then mirror is plane.

b. If the image is of larger size and erect and changes to inverted and smaller size by changing the position of mirror from the object it is concave mirror.

c. If the image is smaller and erect for all positions of mirror from the object, it is a convex mirror.

Question 6:
What is meant by radius of curvature of a spherical mirror? How is it related to the focal length of the mirror?

2 mark; CBSE 2013

Radius of curvature of a spherical mirror is the radius of the sphere of which mirror is a part. It is the distance between pole and centre of curvature of a mirror. The radius of curvature is equal to twice the focal length.

R = 2f

Question 7:
For the same angle of incidence in media P, Q and R, the angles of refraction are 45°, 35° and 15° respectively. In which medium will the velocity of light be minimum? Give reason.

2 mark; CBSE 2012

We know that;

n = c / v = sin i / sin r

∴ v = c (sin r) / sin i

So, for same incident angle, v will be minimum for minimum value of angle r i.e. medium R.

Question 8:
(a) What should be the position of the object, when a concave mirror is to be used:
(i) as a shaving mirror, and
(ii) in torches producing parallel beam of light?

(b) A man standing in front of a mirror finds his image having a very small head and legs of normal size. What type of mirrors are used in designing such a mirror?

2 mark; CBSE 2011

(a)
(i) Face must be placed in between the pole and focus of the mirror so that enlarged, erect image of face can be formed.
(ii) At focus, because rays coming from the focus after reflection will become parallel.

(b) It is a combination of convex mirror (small head) and plane mirror (legs of normal size).

Question 9:
The nature, size and position of image of an object produced by a lens or mirror are as shown below. Identify the lens / mirror (X) used in each case and draw the corresponding complete ray diagram, (size of the object is about half of the image).

3 mark; CBSE 2016

i. Convex lens when object is in between F and C (2F).

ii. Concave mirror when object is in between P and F its enlarged, erected and virtual image is formed.

Question 10:
Define the following terms in the context of a diverging mirror:
(i) Principal focus
(ii) Focal length

3 mark; CBSE 2023

For diverging mirror (convex mirror);

(i) Principal focus: It is the point on the principal axis where rays incident parallel to the principal axis appear to diverge after reflection.

(ii) Focal length: The distance between the pole of a mirror and the principal focus is called focal length.

Question 11:
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 nature and power of each lens.

3 mark; CBSE 2017, 2018

Power of lens is the ability of a lens to converge or diverge light rays passing through it. It is the reciprocal of the focal length.

S.I. Unit: If focal length is measured in metre then unit of power of a lens is Dioptre (D).

Power of first lens:
Focal length = +40 cm.
Focal length is positive hence it is a convex lens.
P = 1 / f = 1 / 40 cm = 100 / 40 m = + 2.5 D.

Power of second lens:
Focal length = -20 cm.
Focal length is negative hence it is a concave lens.
P = 1 / f = 1 / (-20 cm) = 100 / (-20 m) = – 5 D.

Question 12:
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 vacuum.

3 mark; CBSE 2017

Laws of refraction:

a. The incident ray, refracted ray and normal to the point of incidence, all lie in the same plane.
b. The ratio of sine of incident angle and sine of angle of refraction for a given pair of medium is constant.

sin i / sin r = constant

Absolute refractive index of a medium is the ratio of speed of light in air or vacuum and speed of light in the medium.

Absolute refractive index = speed of light in air (or vacuum) / speed of light in medium

Question 13:
Define power of a lens. The focal length of a lens is -10 cm. Write the nature of the lens and find its power. If an object is placed at a distance of 20 cm from the optical centre of this lens, what will be the sign of magnification and nature of image in this case?

5 mark; CBSE 2023

Power of lens is the ability of a lens to converge or diverge light rays passing through it. It is the reciprocal of the focal length.

P = 1 / f

S.I. Unit: If focal length is measured in metre then unit of power of a lens is Dioptre (D).

Given: focal length = -10 cm.
Nature of lens: Focal length is negative hence it is a concave lens.

Power of lens, P = 1 / f = 1 / (-10 cm) = 100 / (-10 m) = – 10 D.

Now, u = – 20 cm, and f = – 10 cm.
So, object is between infinity and optical centre. And because of this, the image formed will be virtual anf erect.

As the image is erect, the sign of magnification will be +ve.