Light – Reflection and Refraction: CBSE Class 10 Science NCERT Solutions

Question 1

Define the principal focus of a concave mirror.

Question 2

The radius of curvature of a spherical mirror is 20 cm. What is its focal length?

Question 3

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

Question 4

Name the mirror that can give an erect and enlarged image of an object.

Question 5

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.

Question 6

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

(a) plane

(b) concave

(c) convex

(d) either plane or convex

Question 7

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.

Question 8

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.

Question 9

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 focused image can be obtained? Find the size and the nature of the image.

Question 10

We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object? Draw a ray diagram to show the image formation in this case.

Question 11

Why do we prefer a convex mirror as a rear-view mirror in vehicles?

Question 12

Name the type of mirror used in the following situations.

(a) Headlights of a car

(b) Side/rear-view mirror of a vehicle

(c) Solar furnace

Support your answer with reason.

Question 13

A concave mirror produces three times magnified (enlarged) real image of object placed at 10 cm in front of it. Where is the image located?

Question 14

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, but the lens is concave

Question 15

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.

Question 16

The magnification produced by a plane mirror is +1. What does this mean?

Question 17

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.

Question 18

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 focused image can be obtained? Find the size and the nature of the image.

Question 19

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?

Question 20

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 × 108 m s−1.

Question 21

Find out, from Table, the medium having highest optical density. Also find the medium with lowest optical density.

Material

medium

Refractive index

Material medium

Refractive

index

Air

1.0003

Canada Balsam

1.53

Ice

1.31

-

-

Water

1.33

Rock salt

1.54

Alcohol

1.36

-

-

Kerosene

1.44

Carbon disulphide

1.63

Fused

quartz

1.46

Dense

flint glass

1.65

Turpentine oil

1.47

Ruby

1.71

Benzene

1.50

Sapphire

1.77

Crown

glass

1.52

Diamond

2.42

Question 22

You are given kerosene, turpentine and water. In which of these does the light travel fastest? Use the information given in Table.

Material

medium

Refractive index

Material medium

Refractive

index

Air

1.0003

Canada Balsam

1.53

Ice

1.31

-

-

Water

1.33

Rock salt

1.54

Alcohol

1.36

-

-

Kerosene

1.44

Carbon disulphide

1.63

Fused

quartz

1.46

Dense

flint glass

1.65

Turpentine oil

1.47

Ruby

1.71

Benzene

1.50

Sapphire

1.77

Crown

glass

1.52

Diamond

2.42

Question 23

The refractive index of diamond is 2.42. What is the meaning of this statement?

Question 24

Which one of the following materials cannot be used to make a lens?

(a) Water

(b) Glass

(c) Plastic

(d) Clay

Question 25

An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the image formed.

Question 26

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.

Question 27

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

(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

Question 28

A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.

Question 29

One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.

Question 30

Define 1 dioptre of power of a lens.

Question 31

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.

Question 32

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, but the lens is concave

Question 33

Find the focal length of a lens of power −2.0 D. What type of lens is this?

Question 34

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?

Question 35

An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the image formed.

Question 36

Find the power of a concave lens of focal length 2 m.

Question 37

Draw ray diagram for concave mirror when

(1) Object at centre of curvature

(2) Object at focus

(3) Object between centre of curvature and focus.

Question 38

Solve:

(1) The speed of light in a transparent medium is 2.4 × 108 m/s. Calculate absolute refractive index of the medium.

(2) The speed of light in water and glass is 2.2 × 108 m/s and 2 × 108 m/s, respectively. What is the refractive index of −

(a) water w.r.t. glass

(b) glass w.r.t. water.

(3) Refractive index of water is 4/3 and speed of light in air is 3×108 m/s. Find speed of light in water.

Question 39

Draw well labelled ray diagrams for image formation by convex lens when

(1) Object is at 2F1

(2) Object between F1 and 2F1

(3) Object between focus F1 and optical centre O.

Question 40

Solve:

(1) An object of size 7 cm is placed at 25 cm in front of concave mirror of focal length 15 cm. At what distance from the mirror should a screen be placed, so that we can get a sharp and clear image. Find nature and size of the image.

(2) An object of height 5 cm is held 20 cm away from converging lens of focal length 10 cm. Find the position, nature and size of the image formed.

(3) An object is placed at a distance of 10 cm from a convex lens of focal length 12 cm. Find the position and nature of the image.

Question 41

Fill in the blanks for convex lens.

f (m) 0.2 0.1
P (D) 2
h1 5 10
h2 −30 −20
M −2 −0.5

Question 42

Find the power of a concave lens of focal length 2 m.

NCERT Question Solutions

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