UNIT

06 LIGHT

Learning Objectives
At the end of this lesson, students will be able to:
‰‰ Apply the laws of reflection for plane mirrors and spherical mirrors.
‰‰ Draw ray diagrams to find the position and size of the image for
spherical mirrors.
‰‰ Distinguish between real and virtual images.
‰‰ Apply the mirror equation to calculate position, size and nature of images for
spherical mirrors.
‰‰ Identify the direction of bending when light passes from one medium to another.
‰‰ Solve problems using Snell’s law.
‰‰ Predict whether light will be refracted or undergo total internal reflection.

TEXT BOOK EXERCISES

I. Choose the correct answer :

1. A ray of light passes from one medium to another medium. Refraction takes place
when angle of incidence is
(a) 0° (b) 45° (c) 90° [Ans : (b) 45°]
2. __________ is used as reflectors in torchlight. 
(a) Concave mirror (b) Plane mirror
(c) Convex mirror  [Ans : (a) Concave mirror]
3. We can create enlarged, virtual images with
(a) concave mirror (b) plane mirror
(c) convex mirror [Ans : (a) concave mirror]
4. When the reflecting surface is curved outwards the mirror formed will be
(a) concave mirror (b) convex mirror
(c) plane mirror  [Ans : (b) convex mirror]

[ 94 ]

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Sura’s  Physics - 9th Std  Unit 06  LIGHT 95

5. When a beam of white light passes through a prism it gets

(a) reflected (b) only deviated
(c) deviated and dispersed [Ans : (c) deviated and dispersed]
6. The speed of light is maximum in  [HY - 2019]
(a) vacuum (b) glass
(c) diamond [Ans : (a) vacuum]
II. State whether true or false. If false, correct the statement :
1. The angle of deviation depends on the refractive index of the glass.
Ans. True.
2. If a ray of light passes obliquely from one medium to another, it does not suffer any
deviation.
Ans. False.
Correct statement : If a ray of light passes obliquely from one medium to another,
it bends away from the normal.
3. The convex mirror always produces a virtual, diminished and erect image of the object.
Ans. True.
4. When an object is at the centre of curvature of concave mirror the image formed will
be virtual and erect.
Ans. False.
Correct statement : When an object is at the centre of curvature of concave mirror
the image formed will be real and inverted.
5. The reason for brilliance of diamonds is total internal reflection of light.
Ans. True.

III. Fill in the blanks :

1. In going from a rarer to denser medium, the ray of light bends ___________.
 [Ans: towards the normal]
2. The mirror used in search light is ___________ . [Ans: concave]
3. The angle of deviation of light ray in a prism depends on the angle of ___________
. [Ans: incidence]
4. The radius of curvature of a concave mirror whose focal length is 5cm is _________.
 [Ans: 10 cm]
5. Large _________ mirrors are used to concentrate sunlight to produce heat in solar
furnaces. [Ans: concave]
IV. Match the following :
Ratio of height of image to height of object. Concave mirror
Used in hairpin bends in mountains. total internal reflection
Coin inside water appearing slightly raised. magnification
Mirage convex mirror
Used as Dentist’s mirror. refraction

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96 Sura’s  Science - 9th Std  Unit 06  LIGHT

Ans.
Ratio of height of image to height of object. magnification
Used in hairpin bends in mountains convex mirror
Coin inside water appearing slightly raised refraction
Mirage total internal reflection
Used as Dentist’s mirror concave mirror

V. Assertion and reason type questions :

Mark the correct choice as:
(a) If both assertion and reason are true and reason is the correct explanation.
(b) If both assertion and reason are true and reason is not the correct explanation.
(c) If assertion is true but reason is false.
(d) If assertion is false but reason is true.
1. Assertion : For observing the traffic at a hairpin bend in mountain paths a plane
mirror is preferred over convex mirror and concave mirror.
Reason :  A convex mirror has a much larger field of view than a plane mirror or a
concave mirror. [Ans : (d) If assertion is false but reason is true]
2. Assertion :  Incident ray is directed towards the centre of curvature of spherical
mirror. After reflection it retraces its path.
Reason : Angle of incidence (i) = Angle of reflection (r) = 0º.
 [Ans : (a) If both assertion and reason are true and
 reason is the correct explanation]
VI. Answer very briefly :
1. According to cartesian sign convention, which mirror and which lens has negative
focal length?
Ans. Concave, mirror is having negative focal length.
2. Name the mirror(s) that can give (i) an erect and enlarged image, (ii) same sized,
inverted image.
Ans. Concave mirror.
3. If an object is placed at the focus of a concave mirror, where is the image formed?
Ans. Infinity as real and inverted.
4. Why does a ray of light bend when it travels from one medium to another?
Ans. A ray of light bend when it travels from one medium to another due to the change in
velocity of light in two different medium.
5. What is the speed of light in vacuum?
Ans. The only medium in which speed of light is equal to that in vacuum is air. Speed of
light in vacuum is 3 × 108 m/s. Armand Fizeau first measured the speed of light.
6. Concave mirrors are used by dentists to examine teeth. Why?
Ans. As concave mirror produces virtual, erect and magnified images when an object
is placed in between focus and pole. So he can clearly gets a magnified view of
cavities.

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Sura’s  Physics - 9th Std  Unit 06  LIGHT 97

VII. Answer briefly :

1. a) Complete the diagram to show how a concave mirror forms the image of
the object.  [HY - 2019]
b) What is the nature of the image?
M

C F

M′

Ans. a)
M

O′ A
I′
P
C O′ F

B
I

M′

b) Real, inverted and magnified.

2. Pick out the concave and convex mirrors from the following and tabulate them.
Rear-view mirror, Dentist’s mirror, Torch-light mirror, Mirrors in shopping
malls, Make-up mirror.
Ans. Concave Mirror Convex Mirror
Dentist’s mirror Rear view mirror
Torch light mirror Mirrors in shopping malls
Make up mirror
3. State the direction of incident ray which after reflection from a spherical mirror
retraces its path. Give reason for your answer.
Ans. Incident ray is directed towards the centre of curvature
Reason : The ray is normal to the spherical mirror, so i = 0,   r = 0
4. What is meant by magnification? Write its expression. What is its sign for real
image and virtual image? [HY - 2019]
Ans. Magnification is the increase in size of an image compared to true size.
height of the image h2
Magnification m =
height of the object h1
−image distance v
=
object distance u
h2 −v
so m = =
h1 u
(a) Negative sign - real image
(b) Positive sign - virtual image

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