6.
Wonders of Light - Part I
C
• Light related phenomena can be studied with mirrors and lenses.
• A mirror is a reflecting surface while a lens is a transparent material.
• Mirrors are mainly of three types : plane mirror, concave or converging mirror,
h
• Lenses are mainly of two types : concave or diverging lens; convex or converging
R
s
negative and image distance (v) is negative in case of real and inverted image
because this type of image is formed on the same side of the mirror as that of the
object. Image distance (v) is positive incase of virtual and erect image because this
type of image is formed behind the mirror.
1
1 1
Mirror formula : +
=
f
v u
on
p
lens.
• Focal length of convex mirror is positive while that of concave mirror is negative.
• Focal length of convex lens is positive while that of concave lens is negative.
• Focal length (f ) of concave mirror is always negative, object distance (u) is always
t
ti
a
convex or diverging mirror.
e
•
r
• M (magnification produced by spherical mirror) = h =
u
1
• h2 = height of the image
• h1 = height of the object
• h2 is negative for inverted image and positive for erect image.
• h1 is always positive.
lu
Figure : Representation of sign conventions.
G
n
c
e
e
qu
Height upward
(+ve)
ni
a
Direction of incident light
distance towards
the left (–ve)
distance towards the
right (+ve)
P
Height downwards
(–ve)
U
l
v
So
at
h2
principal axis
Mirror
• Focal length (f ) of convex lens is always positive, object distance (u) is always
negative and image distance (v) is negative incase of virtual and erect image
because this type of image is formed on the same side of the lens as the object.
Image distance (v) is positive incase of real and inverted images because these
images are formed on the other side of the lens as the object.
• Lens formula =
1
1 1
–
=
f
v u
94
Unique
h2
MCQ's
v
• M (magnification produced by lens) = h =
u
1
1
• P (Power of lens) = f (in metre)
• Unit of power of lens is dioptre
1
l (metre)
R
1 dioptre =
• Cornea, pupil, iris, lens, retina and optic nerve are the main parts of human eye.
on
s
Cornea – light enters the eye.
pupil – light is controlled and regulated.
iris – colour is different for different people.
retina – light sensitive screen.
optic nerve – pass electric signals to the brain.
ti
• Defects of vision : Myopia or near sightedness; Hypermetropia or long sightedness; presbyopia
or old age hypermetropia.
lu
• Myopia – image in front of retina – corrected using concave lens.
• Hypermetropia – image behind retina – corrected using convex lens.
• Presbyopia – suffer, both myopia and hypermetropia – corrected using bifocal lens.
So
• Concave or converging mirror is used in : torches and head lights; flood lights; reflector or
projector lamps; solar devices and furnaces; shaving mirrors and dentists mirror.
• Convex or converging lens is used in : simple microscope, compound microscope; telescope,
e
optical instruments like camera, projector, spectrometer, spectacles etc.
ni
qu
MULTIPLE CHOICE QUESTIONS (MCQ's)
(Each question carries one mark)
MCQ's BASED ON PRACTICALS
Experiment : I. To determine the focal length of a) Concave mirror
1.
2.
U
b) Convex lens by obtaining image of a distinct object.
An object of 10 cm height is placed in front of a plane mirror. The height of the image will be :
a 5 cm
c 15 cm
b 10 cm
d 20 cm
The focal length of concave mirror in the given experimental set up is :
a 10.3 cm
b 11.0 cm
c 11.7 cm
d 12.2 cm
Wonders of Light - Part I
SCIENCE & TECHNOLOGY - S.S.C.
3.
95
A students obtained a sharp image of grill of a window on a screen using a convex lens. For
better results the teacher suggested focusing on a distant tree instead of grill. In which
direction should the lens be moved to get the sharp image of tree?
b behind the screen
c towards the screen
d far away from screen
Inside water, an air bubble behaves :
a always like a concave lens
b always like a convex lens
c always like a flat plate
R
4.
a away from screen
a plane
7.
In a concave mirror light is reflected from :
b outer surface
c spherical surface
d inner and outer surface
What happens to the image of your face as you take a concave mirror away from your face
b becomes bigger
a between focus and centre of curvature
b at focus
c at centre of curvature
d beyond centre of curvature
e
A concave mirror forms enlarged virtual image for the position of the object :
qu
c at the principle focus
13.
ni
b highly magnified
c diminished
d highly diminished
To get the image at the centre of curvature of a concave mirror, the object must be placed :
a between infinity and centre of curvature
b at infinity
c at principal focus
d at the centre of curvature
a centre of curvature
b focus
c pole
d radius of curvature
A ray of light parallel to principal axis after reflection from concave mirror passes through :
If a lens is thicker in the middle than at the edges, it is a :
a bifocal lens
14.
d between pole and principal focus.
U
12.
b between focus and centre of curvature
When the object is at principle focus of a concave mirror, the image formed is :
a magnified
11.
d disappears
When the object is far away from a concave mirror, the image is formed :
a between infinity and centre of curvature
10.
c remains the same
So
a becomes smaller
9.
d both (b) & (c)
a inner surface
slowly?
8.
c concave
ti
6.
b convex
on
Mirror used by a dental surgeon is :
lu
5.
s
d sometimes like a convex lens and sometimes like a concave lens
b diverging lens
c concave lens
d convex lens
A point sized image is obtained by a convex lens when the object is :
a between F1 and 2F1
b at 2F1
c beyond 2F1
Wonders of Light - Part I
d at infinity
96
The focus of a concave mirror lies :
a on the both sides of the mirror
b in front of the mirror
c at the back of the mirror
d on the right side of the mirror
The focal length of a concave mirror is :
a positive
18.
c zero
The focal length of a convex lens is :
a positive
b negative
c always negative
d neither positive nor negative
Which of the following are used in solar furnaces?
a concave lenses
b convex lenses
c concave mirrors
Moderate Level
Distorted images are obtained in
b convex lens
b outer
Diverging mirror is :
a concave
b convex
8.
10.
c inner and outer
d spherical
c plane
d spherical
c focus
d pole
a passes through the focus
b passes parallel to the principal axis
c traces the same path
d passes through optical centre undeviated
Distances parallel to principal axis are measured from the :
a optical centre of the mirror.
b pole of the mirror
c focus of the mirror
d centre of curvature of the mirror
A lens having both surfaces curved inwards is :
a double concave
9.
d plane mirror.
If the incident ray passes through the centre of curvature of a spherical mirror, the
reflected ray :
U
7.
ni
qu
a centre of curvature b optical centre
6.
c spherical mirror
The centre of a spherical mirror is its :
e
5.
d convex lens
To get a convex mirror, which side of a spherical surface is to be polished?
a inner
4.
d convex mirrors
Which of the following forms an arc of a circle?
a concave lens
3.
c concave lens
So
2.
b spherical mirror
lu
a plane mirror
ti
MCQ's BASED ON CHAPTER
1.
d always positive
R
17.
b negative
s
16.
MCQ's
on
15.
Unique
b double convex
c bifocal
d converging lens
A ray of light passes undeviated as it passes through :
a pole of the lens
b focus of the lens
c optical centre of the lens
d centre of curvature of the lens
To get an image of the same size as the object, the object must be placed :
a at 2F1 of a lens
b at 2F2 of a lens
c at F1 of a lens
d beyond 2F1 of a lens
Wonders of Light - Part I
SCIENCE & TECHNOLOGY - S.S.C.
a diminished, virtual and erect
b diminished, real and inverted
c magnified, virtual and erect
d magnified, real and inverted
The ability of a lens to converge or diverge light rays depends on its :
a principal focus
b iris
b iris
d eye lens
c eyelens
d pupil
The focal length of a normal human eye lens in its relaxed position is about :
a 2.5 cm
16.
c pupil
Some people have bluish eyes due to their blue coloured :
a cornea
15.
d optical centre
The amount of light entering the eye is regulated by :
a cornea
14.
c principal axis
R
13.
b focal length
s
12.
The image obtained when the object is placed between focus and optical centre of a convex
lens is :
b 25 cm
c 2.5 m
Hypermetropia is corrected using :
b convex lens
c bifocal lens
ti
a concave lens
on
11.
97
d 25 m
d cylindrical lens
1.
lu
Elementary level
The image formed by a plane mirror cannot be projected on a screen because :
a it is a real image
b it is formed in front of the mirror
So
c reflected rays do not actually meet each other
d reflected rays converge at a point
The image formed by a plane mirror is :
a virtual
3.
b real
qu
d pointed
ni
b solar devices
c table lamps
d head lights
U
b diverging
c converging
d pointed
b behind
c on both sides
d on the left side
b image distance
c object size
d image size
Height of the image is to be taken as positive for :
a real images
10.
c parallel
Which of the following is always negative?
a object distance
9.
b diverging
Where does the focus of a convex mirror lie?
a in front of
8.
d virtual
Which of the following beam of light is used by an ophthalmologist to examine his patient?
a parallel
7.
c real
Diverging beam of light is used in :
a torches
6.
b inverted
Which beam of light is used to concentrate light at a point?
a converging
5.
d inverted
Which of the following image is not obtained on a screen?
a diminished
4.
c magnified
e
2.
b virtual images
c inverted images
d magnified images
Which of the following is a natural optical device?
a lens
b mirror
c eye
Wonders of Light - Part I
d camera
98
11.
12.
Unique
MCQ's
A lens is a/an :
a opaque material
b transparent material
c translucent material
d reflecting material
Which of the following statement is false?
a each lens has two centres of curvature
b every lens has two principal foci
c each spherical mirror has two centres of curvature
13.
Using a convex lens, a well defined image of a distant object is obtained on a screen. The
a the distance between the screen and object
b the distance between the lens and object
c the distance between the screen and lens
Light rays bend through large angles when the focal length of a convex lens is :
b large
b inverted and real
e
b real and erect
c virtual and inverted
ni
qu
b 2.5 m
c 2.5 cm
In myopia, the image of the object is formed :
a on the retina
b in front of the retina
c behind the retina
d on the lens
U
In torches and headlights, the source of light is placed :
a at the centre of curvature of a concave mirror
b at the focus of a concave mirror
c between pole and focus of a concave mirror
d beyond the centre of curvature of a concave mirror
21.
d virtual and erect
For normal human eye, the distance of distinct vision is :
a 25 m
20.
d erect and real
The image formed on the retina of human eye is :
a real and inverted
19.
d metre
Negative sign of magnification shows that the image is :
c erect and virtual
18.
c decimetre
So
b centimetre
a inverted and virtual
17.
d zero
1 dioptre is the power of a lens whose focal length is one :
a millimetre
16.
c medium
lu
a short
15.
ti
d half of the distance between the lens and object
on
s
focal length of this lens is :
14.
R
d every spherical mirror has more than one radii of curvature
In reflecting mirrors for projector lamps, the object is placed :
a at the focus of a concave mirror
b beyond the centre of curvature of a concave mirror
c between pole and focus of a concave mirror
d at the centre of curvature of a concave mirror
Wonders of Light - Part I
d 25 cm
SCIENCE & TECHNOLOGY - S.S.C.
99
Complex Level
The angle of incidence of an incident ray passing through the centre of curvature of a
spherical mirror is :
a 90
4.
Radius of curvature of a spherical mirror is :
b twice larger than its focal length
c thrice larger than its focal length
d larger by 2 cm than its focal length
R
a smaller than its focal length
R
,  R = 2f
2
If the focal length of each a mirror and a lens is + 5 cm then,
a the mirror is concave, but the lens is convex
c both are convex
b both are concave
d the mirror is convex, but the lens is concave.
Which of the following formulae gives height of the image formed by a convex lens?
Hint : f =
s
3.
a h2 =
vh1
u
vh2
v
b h1 =
c h2 =
h2
v
Hint : M = h =
u
1
5.
The value h2 = –3.5 cm shows that the image is :
a enlarged
vh1
v
So

b diminished
c erect
b + 0.025 D
qu
a + 2.5 D
c – 2.5 D
Hint : P =
7.
Focal length of a corrective lens having power + 2D is :
b 0.05 cm
ni
a 0.05 m
c 0.5 m
8.
The negative power of the spectacles indicates the defect of vision :
U
Hint : P =
a myopia
d – 0.025 D
d 0.5 cm
b hypermetropia
c presbyopia
d astigmatism
Since, Rahul's grand father is suffering from presbyopia, he wears spectacles having :
a convex lens
b concave lens
c bifocal lens
d plane lens
The power of a concave lens of focal length 20 cm is :
a + 5D

d inverted
1
f (in metre)

10.
vh 2
u
1
f (in metre)

9.
d h1 =
The power of a convex lens with focal length 40 cm is :
e
6.
d 60
on

c 0
ti
2.
b 45
lu
1.
Hint : P =
b – 5D
c + 0.5D
1
f (in metre)
Wonders of Light - Part I
d – 0.5D
100
Unique
MCQ's
Mysterious Level
1.
The focal length of a spherical mirror with radius of curvature 2.6 cm is :
a 3.2 cm
c 5.2 cm
b 1.3 cm
d 0.3 cm

Hint : f =

Hint : f =
R
2.
R
2
The radius of curvature of a spherical mirror with focal length 0.7 cm is :
a 1.4 cm
c 3.5 cm
b 0.35 cm
d 0.14 cm

Hint : U M =
on
s
3.
R
, R = 2 f
2
If the distance of the object from the origin is 12 cm and the image distance is 60 cm,
magnification produced by the convex lens is :
a + 0.2
c +5
b – 0.2
d – 5

Hint : h2 =
lu
ti
4.
v
u
When a 4 cm tall object is placed at 10 cm away from a converging lens, an image is formed
at a distance of 30 cm on the other side of the lens. The height of the image formed is :
a + 12 cm
c + 15 cm
b – 12 cm
d – 15 cm

Hint :
6.
When a converging lens form an image of –45 cm, the magnification produced is –3.
Therefore, height of the object is :
a + 15 cm
c + 30 cm
b – 15 cm
d – 30 cm
e
ni
qu
7.
1 1
1
= +
u
f
v
h2
h2
Hint : m = h
 h1 =
m
1
If the magnification produced by an object of height 10 cm due to a spherical mirror is
– 0.5, height of the image formed is :
a – 5 cm
c + 2 cm
b + 5 cm
d – 2 cm
U

So
5.
v
× h1
u
An object is placed at a distance of 15 cm from a convex lens of focal length 20 cm. The
position of the object is :
a – 60 cm
c – 90 cm
b + 60 cm
d + 90 cm
8.
h2
Hint : M = h
h2 = M  h1
1
An object of size 5 cm is placed at 15 cm in front of concave mirror of focal length 12 cm. To
get a sharp and clear image, at what distance from the mirror a screen should be placed?
a 60 cm
c 20 cm
b 30 cm
d 50 cm

Hint :

1
1 1
+ =
f
v u

1 1
1
–
=
f
u
v
Wonders of Light - Part I
SCIENCE & TECHNOLOGY - S.S.C.
101
Answers
MCQ's BASED ON PRACTICAL SKILLS
-
b
2
-
b
3
-
c
4
-
b
5
-
c
6
-
a
7
-
b
8
-
b
9
-
d
10
-
b
11
-
d
12
-
b
13
-
d
14
-
d
15
-
b
16
-
b
17
-
a
18
-
c
MCQ's BASED ON CHAPTER
2
-
c
3
6
- c
7
-
b
8
11
- c
12
-
b
16
- b
1
- c
2
-
6
- c
11
- b
16
- b
21
- d
4
-
b
5
-
d
-
a
9
-
c
10
-
a
-
c
14
-
b
15
-
a
lu
13
3
So
a
-
d
4
-
a
5
-
c
7
-
b
8
-
a
9
-
b
10
-
c
12
-
c
13
-
c
14
-
a
15
-
d
17
-
a
18
-
d
19
-
b
20
-
b
- c
2
-
b
3
-
c
4
-
a
5
-
d
6
- a
7
-
c
8
-
a
9
-
c
10
-
b
4
-
b
5
-
a
1
- b
2
-
a
3
-
d
6
- a
7
-
a
8
-
a
U
Mysterious Level
b
1
ni
Complex Level
-
ti
- b
qu
Elementary Level
1
e
Moderate Level
on
s
R
1

Wonders of Light - Part I