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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