Class 10 Science

Chapter 9 Solutions — Light — Reflection and Refraction

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Solutions

Overview

Step-by-step NCERT solutions for Light — Reflection and Refraction (Chapter 9, CBSE Class 10 Science) — every question and answer worked out in full, not just the final result. You can also read the Light — Reflection and Refraction textbook chapter.

Solved

What these solutions cover

All 26 questions in Light — Reflection and Refraction are solved in the PDF. Here's what's inside, exercise by exercise:

Reflection of Light and Spherical Mirrors

Define the principal focus of a concave mirror.
The radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Name a mirror that can give an erect and enlarged image of an object.
Why do we prefer a convex mirror as a rear-view mirror in vehicles?

Image Formation by Spherical Mirrors and Mirror Formula

Find the focal length of a convex mirror whose radius of curvature is 32 cm.
A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located?

Refraction of Light

A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?
Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 x 10⁸ m/s.
Find out, from Table 10.3, the medium having highest optical density. Also find the medium with lowest optical density.
You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in Table 10.3.
The refractive index of diamond is 2.42. What is the meaning of this statement?

Image Formation by Lenses, Lens Formula and Power

Define 1 dioptre of power of a lens.
A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. 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.
Find the power of a concave lens of focal length 2 m.

Exercise

1.
Which one of the following materials cannot be used to make a lens?
- (a) Water
- (b) Glass
- (c) Plastic
- (d) Clay
2.
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?
- (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
3.
Where should an object be placed in front of a convex lens to get a real image of the size of the object?
- (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
4.
A spherical mirror and a thin spherical lens have each a focal length of -15 cm. The mirror and the lens are likely to be
- (a) both concave
- (b) both convex
- (c) the mirror is concave and the lens is convex
- (d) the mirror is convex and the lens is concave
5.
No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be
- (a) plane only
- (b) concave only
- (c) convex only
- (d) either plane or convex
An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image.
The magnification produced by a plane mirror is +1. What does this mean?
An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size.
An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focussed image can be obtained? Find the size and the nature of the image.
Find the focal length of a lens of power -2.0 D. What type of lens is this?
A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?
How are power and focal length of a lens related? What is the SI unit of power of a lens?

Sample

Worked examples

3 questions from Light — Reflection and Refraction, solved step by step.

Reflection of Light and Spherical Mirrors

Define the principal focus of a concave mirror.

A number of rays parallel to the principal axis are incident on a concave mirror. After reflection, these rays converge (meet) at a specific point on the principal axis.

This point is called the principal focus (F) of the concave mirror.

The principal focus of a concave mirror is a point on the principal axis at which rays of light incident parallel to the principal axis actually meet (converge) after reflection from the mirror.

Note: The principal focus of a concave mirror is a real point (rays actually pass through it), unlike a convex mirror where the focus is virtual.

Answer: The principal focus of a concave mirror is a point on the principal axis at which rays of light parallel to the principal axis actually converge after reflection from the mirror.

Image Formation by Spherical Mirrors and Mirror Formula

Find the focal length of a convex mirror whose radius of curvature is 32 cm.

Given:

Radius of curvature, R = 32 cm

We know the relationship between focal length and radius of curvature:

f = R / 2

Substituting the value:

f = 32 / 2

f = 16 cm

Since it is a convex mirror, the focal length is positive (by sign convention).

Therefore, the focal length of the convex mirror is +16 cm.

Answer: The focal length of the convex mirror is 16 cm.

Refraction of Light

A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?

The light ray bends towards the normal when it enters from air into water.

This is because water is optically denser than air. When a ray of light travels from a rarer medium (air) to a denser medium (water), it slows down.

According to the laws of refraction (Snell's law), when light enters a denser medium:

→ The speed of light decreases

→ The refracted ray bends towards the normal

→ The angle of refraction (r) is less than the angle of incidence (i)

This happens because the refractive index of water (approximately 1.33) is greater than the refractive index of air (approximately 1.0).

Answer: The light ray bends towards the normal because water is optically denser than air, and light slows down when entering a denser medium.

All 26 questions in Light — Reflection and Refraction are solved step by step in the Solutions PDF above.

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Chapter 9 Solutions — Light — Reflection and Refraction

Overview

What these solutions cover

Reflection of Light and Spherical Mirrors

Image Formation by Spherical Mirrors and Mirror Formula

Refraction of Light

Image Formation by Lenses, Lens Formula and Power

Exercise

Worked examples

Define the principal focus of a concave mirror.

Find the focal length of a convex mirror whose radius of curvature is 32 cm.

A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?

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