Summary

Light is a form of energy which can be reflected either by a plane mirror or spherical mirror. Depending upon the

type of mirror used for reflection of light the use of reflecting surface also varies.

Light

Light is an external cause responsible for sensation of vision.

Reflection

The phenomenon by which a ray of light changes the direction of propagation when it strikes a boundary

between different media through which it cannot pass is described as the reflection of light.

Laws of Reflection

The incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.

The angle of incidence is equal to the angle of reflection.

Concave Mirror

Concave mirror is a spherical mirror whose reflecting surface is towards the centre of the sphere of which the

mirror is a part.

Convex Mirror

Convex mirror is a spherical mirror whose reflecting surface is away from the centre of the sphere of which the

mirror is a part.

Centre of Curvature (C)

Centre of curvature (C) is the centre of the sphere of which the spherical mirror forms a part.

Radius of Curvature (R)

Radius of curvature (R) is the radius of the sphere of which the mirror is a part.

Linear Aperture

Linear aperture is the distance between the extreme points on the periphery of the mirror.

Pole

Pole is the mid point of the aperture of the spherical mirror.

Principal Axis

Principal axis is the straight line passing through the pole and the centre of curvature of a spherical mirror.

Principal Focus

The rays of light parallel to the principal axis of a mirror after reflection, either pass through a point (in the case of

a concave mirror) or appear to diverge from a point (in the case of a convex mirror) on the principal axis and this

point is referred to as the principal focus of the mirror.

Focal Length

Focal length is the distance between the pole and the focus of a mirror.

Focal length of a mirror is equal to half the focal length of the mirror or f = R/2.

Mirror Formula

Magnification

Magnification is the ratio of the height of the image to the height of the object.

Question (1): Define reflection of light.

Answer:  The phenomenon by which a ray of light changes the direction

of propagation when it strikes a boundary between different media

through which it cannot pass is described as the reflection of light.

Question (2): Name the two types of

reflection.

Answer:  The two types of reflection are regular reflection and irregular or

diffused reflection.

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Question (3): What is meant by diffused

reflection?

Answer:  In this type of reflection the different portions of the surface

reflect the incident light in different directions and no definite image is

formed but the surface becomes visible.

Question (4): Name the phenomenon due to

which we see the non-luminous objects.

Answer:  It is due to diffused or irregular reflection of light that we see the

non-luminous objects.

Question (5): While looking at your image in

a large plane mirror, if you touch your left ear

it appears as if your image has touched the

right ear. Why?

Answer:  It is due to lateral inversion that we feel that the image has

touched the right ear.

Question (6): Name any one device based

upon the principle of reflection of light by

plane mirrors.

Answer:  Periscope is a device based on the principle of reflection of

light.

Question (7): Define magnification.

Answer:  The ratio of the height of the image to the height of the object is

called the linear magnification.

For a real image u and v are negative and the magnification is negative.

Negative magnification means the image is inverted. On the other hand

for a virtual image u is negative and v is positive and hence the

magnification is positive, i.e., the image is erect.

Question (8): What type of surface causes

regular reflection?

Answer:  Smooth polished surfaces cause regular reflection.

Question (9): Define a) Centre of curvature

b) Pole of a spherical mirror

Answer:  Centre of curvature is the centre of the sphere of which the

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Reflection of Light

Introduction

Nature of Light

Rectilinear Propagation of

Light

Reflection of Light

Reflection of Light by a

Plane Surface

Verification of the Laws of

Reflection

Nature of the Image

Formed by a Plane

Reflecting Surface

Formation of Image by a

Plane Mirror - Ray

Diagrams

Spherical Mirrors

Sign Convention for

Spherical Mirrors

Formation of Images by

Spherical Mirrors

spherical mirror forms a part. Pole of a mirror is the midpoint of the

aperture of the spherical mirror.

Question (10): What kind of mirror is used in

a car to view the traffic behind it?

Answer:  Convex mirror is used in a car to view the traffic behind it.

Question (11): For which position of an

object does a concave mirror produce an

inverted, magnified and real image?

Answer:  When the object is placed between the focus and the centre of

curvature of the concave mirror, the image formed will be inverted,

magnified and real.

Question (12): Name the spherical mirror

which can produce a real and diminished

image of an object.

Answer:  Concave mirror can produce a real and diminished image of an

object.

Question (13): Give the nature, position and

size of the image formed by a concave mirror

when an object is placed between focus and

pole of the mirror.

Answer:  When an object is placed between the focus and pole of the

mirror, the image formed will be virtual, erect and magnified. It will be

formed behind the mirror.

Question (14): List out any two uses of

concave mirror.

Answer:  Concave mirrors are used

1) as reflectors in the head lights of cars, search lights, etc. to obtain a

parallel beam of light.

2) by dentists to focus light on the patient's tooth to be examined.

Question (15): What is the nature of the ray

of light incident on a concave mirror, if it

passes through the focus of the mirror after

reflection?

Answer:  The ray of light is parallel to the principal axis

Formation of Images by a

Concave Mirror

Uses of Concave Mirror

Formation of Images by a

Convex Mirror

Formation of Image in a

Convex Mirror

Mirror Formula (Concave

Mirror)

Magnification

Summary

Question and Answers

Multiple Choice Questions

Question (16): What is the mirror formula for

a concave mirror?

Answer:  The relationship between the focal length of the mirror (f), the

distance of the object from the pole of the mirror (u) and the distance of

the image from the pole of the mirror (v) is given by:

This is called the mirror formula.

Question (17): What is the nature of the

image formed by a concave mirror, if the

magnification produced by it is negative?

Answer:  The image formed will be inverted and real as the magnification

is negative.

Question (18): State the laws of reflection.

Answer:  The laws of reflection are:

1) The incident ray, the reflected ray and the normal drawn at the point of

incidence all lie in the same plane.

2) The angle of incidence is equal to the angle of reflection.

Question (19): List out the characteristics of

the image formed by a plane mirror.

Answer:  The image formed by a plane mirror is:

1) erect

2) virtual

3) of the same size as the object

4) the image is at the same distance behind the mirror as the object is in

front of it

5) laterally inverted

Question (20): Distinguish between concave

and convex mirrors.

Answer: 

Convex Mirror Concave Mirror

The focus of a convex mirror

lies behind the mirror

The focus of a concave

mirror is in front of the

mirror

The focus of a convex mirror

is virtual as the rays of light

after reflection appear to

The focus of a concave

mirror is real as the rays of

light after reflection

come from the focus converge at the focus

Question (21): Give any four uses of plane

mirror.

Answer:  A plane mirror is used

1. as a looking glass to see our image

2. by interior designers to create an illusion of depth

3. to fold light in a periscope and other optical instruments

4. to build a kaleidoscope, an interesting toy

Question (22): The radius of curvature of a

mirror is + 40 cm.

1) Identify the mirror

2) What is the focal length of the mirror?

Answer:  The mirror is a convex mirror.

The focal length

Question (23): Two concave mirrors 'A' and

'B' are made from two spheres of radii 30 cm

and 32 cm respectively. Which of the two

concave mirrors will have larger focal length?

What will be the focal length of the concave

mirrors 'A' and 'B'?

Answer:  The mirror 'B' will have a larger focal length.

The focal length of the mirror 'B'

The focal length of the mirror 'A'

Question (24): You are given two mirrors 'A'

and 'B' of focal lengths +10 cm and -20 cm

respectively. Which one will you use as a

converging mirror? Why?

Answer:  Mirror B with focal length -20 cm is used as a converging mirror

as the parallel rays of light after reflection meet at a point on the principal

focus and for a concave mirror according to sign convention the focal

length is negative.

Question (25): Why is a convex mirror used

as a rear-view mirror in automobiles?

Answer:  A convex mirror is used as a rear-view mirror because of the

following reasons:

1) A convex mirror always forms an erect image of an object

2) It forms a diminished image and as a result images of large number of

objects can be seen in the mirror at the same time.

Question (26): You are given three mirrors of

equal size. How will you identify them as

convex, concave and plane mirror?

Answer:  Bring each mirror one by one close to an object and observe

the image formed in it. If the image is :

1) of the same size as that of the object and erect, the mirror is a plane

mirror.

2) large and erect, it is a concave mirror.

3) highly diminished and erect, it is a convex mirror.

Question (27): An object is placed at a

distance of 5 m from a convex mirror of radius

of curvature 20 cm where is the image formed

and what is its nature?

Answer:  For a convex mirror f and R are positive and u is negative.

Distance of the object from the mirror (u) = -5 m = -500 cm

Distance of the image from the mirror (v) = ?

Radius of curvature = 20 cm

Focal length =

= 10 cm

Mirror formula is

= 9.8 cm

The image is formed 9.8 cm behind the mirror. The image formed is virtual

and erec

Question (28): What is the position of an

image when an object is placed at a distance

of 20 cm from a concave mirror of focal length

20 cm?

Answer:  For a concave mirror both u and f are negative

Distance between the object and the mirror (u)

= -20 cm Focal length of the mirror (f)

= -20 cm Mirror formula is

Thus, the image will be formed at infinity.

Question (29): An object is placed at

distance of 10cm from the pole of a mirror,

and the image of the object is formed at a

distance of 30 cm from the mirror on the

same side as the object. Is the mirror concave

or convex? What is its focal length?

Answer:  The image in this case is real as it is in front of the mirror. Only

concave mirror gives a real image hence it is a concave mirror.

The distance between the object and the mirror u = -10 cm

The distance between the image and the mirror v = -30cm Focal length f =

?

= - 7.5 cm

The focal length is negative as the mirror is concave mirror.

Question (30): Explain with the help of ray

diagrams the rules to be followed when

drawing ray diagrams in the case of a

concave mirror.

Answer:  1) A ray of light parallel to the principal axis after reflection from

a concave mirror passes through its focus.

2) A ray of light passing through the focus of a concave mirror after

reflection emerges parallel to the principal axis.

3) A ray of light passing through the centre of curvature of a concave

mirror retraces its path after reflection as the ray passing through the

centre of curvature acts as a normal to the spherical mirror.

4) A ray of light which strikes the mirror at its pole gets reflected according

to the law of reflection.

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Question (31): Draw a ray diagram to show

the position, nature and size of the image

formed by a concave mirror when the object

is placed beyond the centre of curvature of

the mirror.

Answer:  Object is placed beyond C

The image formed is between F and C. It is a) inverted b) diminished and

c) real

Question (32): Draw a ray diagram to show

the position, nature and size of the image

formed by a convex mirror when the object is

placed at infinity.

Answer:  Object placed at infinity

The image is formed at the focus of the mirror. It is virtual, diminished and

erect.

Question (33): The focal length of a concave

mirror is 15 cm. If the object is placed at a

distance of 20 cm from the pole of the mirror,

where will the image be formed? Draw the ray

diagram to show the formation of the image.

Answer:  Focal length (f) = - 15 cm

The distance between the object and the mirror (u) = - 20 cm

The distance between the image and the mirror (v) = ?

V = -60 cm

The image is formed 60 cm away from the pole of the mirror that is, on the

same side of the mirror where the object is placed.

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Question (34): The image formed by a

convex mirror of focal length 30 cm is 1/4th

size of the object. What is the distance of the

object from the mirror?

Answer:  Focal length = +30 cm

The distance between the object and the mirror (u) = ?

The distance between the image and the mirror (v) = ?

Let the size of the object be equal to 1.

Given size of the image = 1/4 size of the object.

ho = 1

But we know that

u = -90 cm

The distance of the object from the mirror = 90 cm

Question (35): An object 6 cm long is placed

1 metre in front of a concave mirror of 10 cm

focal length. Find the nature and the size of

the image.

Answer:  u = -1m = -100cm f = -10cm

v = ?

= -11.11cm

The image is real and inverted and 11.11 cm from the mirror.

hI = -0.666cm

Size of the image = 0.666 cm

Negative sign indicates that the image is real and inverted.

Question (36): A candle is held 3 cm away

from a concave mirror whose radius of

curvature is 24 cm. Where is the image

formed? What is the nature of the image?

Answer:  Radius of the curvature = -24 cm

Focal length

= -12 cm

Distance between the object and the mirror u = -3 cm

Distance between the image and the mirror v = ?

= 4 cm

Magnification

= + 1.33

The image is 4 cm behind the mirror and it is virtual, magnified and erect

Question (37): How far should one hold an

object from a concave mirror of focal length

40 cm so as to get virtual image twice the size

of the object?

Answer:  Focal length = -40 cm

Let the height of the object be equal to 1

Then, the height of the image = 2

(Since the mirror is a concave mirror the image formed will be erect and

virtual.)

v = +2u

The object should be held at 60 cm from the pole of the mirro

Question (38): An object is kept at a distance

of 5 cm in front of a convex mirror of focal

length 10 cm. Calculate the position and

magnification of the image.

Answer:  Distance between the object and the mirror u = -5 cm

Focal length f = +10 cm

Distance between the image and the mirror v = ?

= 3.33cm

The image is formed 3.33 cm behind the mirror.

Magnification

= +0.67

As the magnification is +ve, the image formed is virtual and erec

Question (39): Describe an experiment to

show that in a homogeneous transparent

medium light travels in a straight line.

Answer:  In a homogeneous transparent medium light travels in a straight

line and this is known as rectilinear propagation of light. This can be

demonstrated by the following experiment:

Take three cardboards A, B and C and make a pinhole at their centre.

Place a candle on one side and arrange the cardboards in such a way

that the three pinholes and the candle flame are in a straight line. Light the

candle, then the candle flame will be visible through the pinhole of the

cardboard C. Now slightly displace any one of the cardboards and try to

see the flame through the pinhole of the cardboard C. The flame will not

be visible. From this it is clear that light travels in a straight line.

Question (40): State the laws of reflection

and verify them.

Answer:  Laws of reflection:

1) The incident ray, the reflected ray and the normal at the point of

incidence lie in the same plane.

2) The angle of incidence is equal to the angle of reflection.

Verification of the Laws of Reflection

1) Fix a sheet of white paper on a drawing board.

2) Draw a line MM' on it and mark a point O at the centre of the line and a

normal ON is drawn on MM'.

3) Draw IO, the incident ray in such a way that is less than 900.

4) Place a strip of plane mirror vertically with its silvered surface on MM'.

5) Fix two pins P and Q on the incident ray IO.

6) Fix P' and Q' on other side of the normal, to represent the reflected

image of P and Q, as observed in the mirror as P' and Q'

7) Remove the pins and draw a line OR passing through P' and Q' to

represent the reflected ray.

8) Measure the angle of incidence and the angle of reflection

.

9) and P, Q, P', Q', N and O lie in the same plane, thus

verifying the laws of reflection.

10) Repeat the experiment for different measures of angles of incidence.

The laws of reflection are found to be always true for different values of

angle of incidence.

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Question (1): Light is a form of energy produced by a ______.

1. luminous object

2. transparent object

3. non-luminous object

4. opaque object

Ans:  1

Question (2): An example for non-luminous object is ___________.

1. a candle

2. the sun

3. an electric bulb

4. the moon

Ans:  4

Question (3): The phenomenon by which the incident light falling on a surface is sent back into the same medium is known as ________.

1. polarization

2. reflection

3. refraction

4. absorption

Ans:  2

Question (4): When light is incident on a polished surface ___________ reflection takes place.

1. regular

2. irregular

3. diffused

4. normal

Ans:  1

Question (5): An object becomes invisible when it undergoes ______ reflection.

1. regular

2. irregular

3. diffused

4. normal

Ans:  1

Question (6): According to the laws of reflection,

1. i = r

2. i > r

3. r > i

4.

Ans:  1

Question (7): The image formed by a plane mirror is always _______.

1. real and erect

2. virtual and erect

3. real and inverted

4. virtual and inverted

Ans:  2

Question (8): The centre of the sphere of which the spherical mirror forms a part is called ____________.

1. centre of curvature

2. focus

3. pole

4. vertex

Ans:  1

Question (9): The focus of a concave mirror is ________.

1. real

2. virtual

3. undefined

4. at the pole

Ans: 2

Question (10): A converging mirror is known as ________.

1. convex mirror

2. plane mirror

3. concave mirror

4. cylindrical mirror

Ans:  3

Question (11): The relation between the focal length and radius of curvature of a mirror is _______.

1.

2. R + 2 = f

3. f = R/2

4. f = 2 R

Ans:  3

Question (12): Radius of curvature of a concave mirror is always _____ to the mirror.

1. parallel

2. perpendicular

3. inclined at 60o

4. inclined at 45o

Ans:  2

Question (13): An image formed by a convex mirror is always ________.

1. virtual, erect and diminished

2. virtual, real and magnified

3. real, inverted and diminished

4. real, erect and magnified

Ans:  1

Question (14): If the image formed by a concave mirror is virtual, erect and magnified, then the object is placed __________.

1. between the pole of the mirror and the focus

2. beyond the centre of curvature

3. at the centre of curvature

4. at the focus

Ans:  1

Question (15): Dentists use a _____________ to focus light on the tooth of a patient.

1. concave mirror

2. convex mirror

3. plane mirror

4. cylindrical mirror

Ans:  1

Question (16): An object is placed 1.5 m from a plane mirror. How far is the image from the

person?

1. 3 m

2. 1.5 m

3. 2 m

4. 1 m

Ans:  1

Question (17): An object placed 2m from a plane mirror is shifted by 0.5 m away from the mirror. What is the distance between the object and its image?

1. 2 m

2. 1.5 m

3. 5 m

4. 3 m

Ans:  3

Question (18): What is the value of in the following diagram?

1. 30o

2. 45o

3. 90o

4. 60o

Ans:  4

Question (19): What is the angle between the incident and reflected rays when a ray of light is incident normally on a plane mirror?

1. 90o

2. 45o

3. 180o

4. 0

Ans:  4

Question (20): Name the type of image that can be obtained on a screen.

1. Virtual

2. Real

3. Diverging

4. Converging

Ans:  2

Question (21): A ray of light is incident on a plane mirror and the angle of incidence is 25o. What is the angle of reflection?

1. 0

2. 50o

3. 90o

4. 25o

Ans:  4

Question (22): A ray of light is incident on a plane mirror and the angle of reflection is 50o. Calculate the angle between the incident ray and the reflected ray.

1. 50o

2. 25o

3. 90o

4. 100o

Ans:  4

Question (23): Which of the following is used to make a periscope?

1. Concave mirror

2. Convex mirror

3. Plane mirror

4. Lens

Ans:  3

Question (24): Which mirror has a wider field of view?

1. Convex mirror

2. Concave mirror

3. Plane mirror

4. Cylindrical mirror

Ans:  1

Question (25): The focal length of a concave mirror is 15 cm. What is its radius of curvature?

1. 15 cm

2. 30 cm

3. 7.5 cm

4. 45 cm

Ans:  2

Question (26): The focal length of a mirror is 15 cm. Identify the type of mirror.

1. Concave mirror

2. Plane mirror

3. Convex mirror

4. Cylindrical mirror

Ans:  3

Question (27): A ray of light passing through the _______ retraces its path.

1. focus

2. centre of curvature

3. pole

4. vertex

Ans:  2

Question (28): When an object is placed at the focus of a concave mirror, the image will be formed at ________.

1. infinity

2. focus

3. centre of curvature

4. pole

Ans:  1

Question (29): Butter paper is an example for _______ object.

1. a transparent

2. a translucent

3. an opaque

4. a luminous

Ans:  2

Question (30): An object of size 2.0 cm is placed perpendicular to the principal axis of a concave mirror. The distance of the object from the mirror equals to the radius of curvature. The size of the image will be ______________.

1. 0.5 cm

2. 1.5 cm

3. 1.0 cm

4. 2.0 cm

Ans:  4

Question (31): If an incident ray passes through the centre of curvature of a spherical mirror, the reflected ray will __________________.

1. pass through the focus

2. pass through the centre of curvature

3. pass through the pole

4. retrace its path

Ans:  4