Lesson: Light Reflection and Refraction
Which of the following can make a parallel beam of light when
light from a point source is incident on it?
(a)
Concave mirror as well as convex lens
(b) Convex
mirror as well as concave lens
(c) Two
plane mirrors placed at 90° to each other
(d)
Concave mirror as well as concave lens
Solution
a
Question: 2
A 10-mm long awl pin is placed vertically in front of a concave
mirror. A 5 mm long image of the awl pin is formed at 30 cm in front of the
mirror. The focal length of this mirror is:
(a) 30
cm
(b) 20
cm
(c) 40
cm
(d) 60
cm
Solution
b
Question: 3
Under which of the following conditions a concave mirror can form
an image larger than the actual object?
(a) When
the object is kept at a distance equal to its radius of curvature
(b) When
object is kept at a distance less than its focal length
(c) When
object is placed between the focus and centre of curvature
(d) When
object is kept at a distance greater than its radius of curvature
Solution
c
Question: 4
In given below figure shows a ray of light as it travels from
medium A to medium B. Refractive index of the medium B relative to medium A is:
(a)
(b)
(c)
(d)
Solution
a
Question: 5
A light ray enters from medium A to medium B as shown in the
following figure. The refractive index of medium B relative to A will be
(a) Greater than unity
(b) less
than unity
(c) Equal
to unity
(d) Zero
Solution
b
Question: 6
Beams of light are incident through the holes A and B and emerge
out of box through the holes C and D respectively as shown in the figure. Which
of the following could be inside the box?
(a) A rectangular glass slab
(b) A
convex lens
(c) A
concave lens
(d) A
prism
Solution
a
Question: 7
A beam of light is incident through the holes on side A and
emerges out of the holes on the other face of the box as shown in the given
below figure. Which of the following could be inside the box?
(a) Concave lens
(b)
Rectangular glass slab
(c) Prism
(d) Convex
lens
Solution
d
Question: 8
Which of
the following statements is true?
(a) A
convex lens has 4 dioptre powers having a focal length 0.25 m
(b) A
convex lens has 4 dioptre powers
having a focal length 0.25 m
(c) A
concave lens has 4 dioptre powers having a focal length 0.25 m
(d) A
concave lens has 4 dioptre powers
having a focal length 0.25 m
Solution
a
Question: 9
Magnification produced by a rear-view mirror fitted in vehicles:
(a) is
less than one
(b) is
more than one
(c) is
equal to one
(d) Can be more than or less than one depending upon the position of
the object in front of it.
Solution
a
Question: 10
Rays from Sun converge at a point 15 cm in front of a concave
mirror. Where an object should be placed so that size of its image is equal to
the size of the object?
(a) 15 cm
in front of the mirror
(b) 30 cm
in front of the mirror
(c) Between
15 cm and 30 cm in front of the mirror
(d) More
than 30 cm in front of the mirror
Solution
b
Question: 11
A full-length
image of a distant tall building can definitely be seen by using:
(a) A
concave mirror
(b) A
convex mirror
(c) A
plane mirror
(d) Both
concave as well as plane mirror
Solution
b
Question: 12
In
torches, search lights and headlights of vehicles the bulb is placed:
(a) Between
the pole and the focus of the reflector
(b) Very
near to the focus of the reflector
(c) Between
the focus and centre of curvature of the reflector
(d) At the
centre of curvature of the reflector
Solution
b
Question: 13
The laws
of reflection hold good for:
(a) Plane
mirror only
(b) Concave
mirror only
(c) Convex
mirror only
(d) All
mirrors irrespective of their shape
Solution
d
Question: 14
The path of a ray of light coming from air passing through a
rectangular glass slab traced by four students are shown as A, B, C and D in the
given figure. Which one of them is correct?
(a) A
(b) B
(c) C
(d) D
Solution
b
Question: 15
You are given water, mustard oil, glycerin and kerosene. In which of
these media a ray of light incident obliquely at same angle would bend the
most?
(a)
Kerosene
(b) Water
(c)
Mustard oil
(d) Glycerin
Solution
d
Question: 16
Which of the following ray diagrams is correct for the ray of
light incident on a concave mirror as shown in figure X?
(a) Fig. A
(b) Fig. B
(c) Fig. C
(d) Fig. D
Solution
d
Question: 17
Which of the following ray diagrams is correct for the ray of
light incident on a lens shown in the following figure?
(a) Fig. A
(b) Fig. B
(c) Fig. C
(d) Fig. D
Solution
a
Question: 18
A child is standing in front of a magic mirror. She finds the
image of her head bigger, the middle portion of her body of the same size and
that of the legs smaller. The following is the order of combinations for the
magic mirror from the top.
(a) Plane,
convex and concave
(b)
Convex, concave and plane
(c)
Concave, plane and convex
(d)
Convex, plane and concave
Solution
c
Question: 19
In which of the following, the image of an object placed at
infinity will be highly diminished and point sized?
(a) Concave mirror only
(b) Convex mirror only
(c) Convex lens only
(d) Concave mirror, convex mirror, concave lens and convex lens
Solution
d
Short
answer Questions
Question: 20
Identify the device used as a spherical mirror or lens in following
cases, when the image formed is virtual and erect in each case.
(a) Object is placed between device and its
focus, image formed is enlarged and behind it.
(b) Object is placed between the focus and
device, image formed is enlarged and on the same side as that of the object.
(c) Object is placed between infinity and
device, image formed is diminished and between focus and optical centre on the
same side as that of the object.
(d) Object is placed between infinity and
device, image formed is diminished and between pole and focus, behind it.
Solution
(a) Concave mirror
(b) Convex lens
(c) Concave lens
(d)
Convex mirror
Question: 21
Why does a light ray incident on a rectangular glass slab immersed
in any medium emerges parallel to itself? Explain using a diagram.
Solution
The
refraction of light in a glass slab can be shown as given below.
On passing
through a rectangular glass slab, a ray of light bends first at the air-glass
interface and then again at the glass- air interface.
The angle
of refraction by first interface is equal to the angle of incidence at the
second interface, i.e. .
The final
emergent ray is always parallel to the incident ray and is displaced through a
distance.
Thus
Question: 22
A pencil when dipped in water in a glass tumbler appears to be bent
at the interface of air and water. Will the pencil appear to be bent to the
same extent, if instead of water we use liquids like, kerosene or turpentine?
Support your answer with reason.
Solution
No,
the bending of light will be different in different mediums since each medium
has a different refractive index. The amount of light refracted or bent depends
on the refractive index of the second medium. Since the
refractive index of kerosene or turpentine is more than that of water, the pencil
will appear to be more bent.
Question: 23
How is the refractive index of a medium related to the speed of light?
Obtain an expression for refractive index of a medium with respect to another
in terms of speed of light in these two media?
Solution
Refractive index of medium is the
ratio of the speed of light in vacuum (or air) to the speed of light in the medium.
So, the refractive index,
Let one of the medium be .
Its refractive index is
Let
the other medium be .
Its refractive index is
Question:
24
Refractive index of diamond with respect to glass is 1.6 and absolute
refractive index of glass is 1.5. Find out the absolute refractive index of
diamond.
Solution
Given
that, the refractive index of diamond with respect to glass is,
The
absolute refractive index of glass is,
The
absolute refractive index of diamond is,
Therefore,
Question: 25
A convex lens of focal length 20 cm can produce a magnified
virtual as well as real image. Is this a correct statement? If yes, where shall
the object be placed in each case for obtaining these images?
Solution
The given statement is
correct.
Magnified virtual
image
For the statement to
be correct, the object has to be placed within 20 cm from the lens in the first
case.
Magnified
real image
For the statement to
be correct, the object has to be placed between 20 cm and 40 cm.
Question: 26
Sudha finds out that the sharp image of the window pane of her science
laboratory is formed at a distance of 15 cm from the lens. She now tries to
focus the building visible to her outside the window instead of the window pane
without disturbing the lens. In which direction will she move the screen to
obtain a sharp image of the building? What is the approximate focal length of
this lens?
Solution
Sudha should move the
screen towards the lens to obtain a clear image of the building because as the
object distance increases, the image distance decreases.
The approximate focal
length of this lens will be 15 cm.
Question: 27
How are power and focal length of a lens related? You are provided
with two lenses of focal length 20 cm and 40 cm respectively. Which lens will
you use to obtain more convergent light?
Solution
The power of a lens is
inversely proportional to its focal length.
The
lens of focal length 20 cm will have more power than the lens of focal length
of 40 cm.
Therefore,
the lens having focal length of 20 cm will provide more convergence.
Question: 28
Under what
condition in an arrangement of two plane mirrors, incident ray and reflected
ray will always be parallel to each other, whatever may be angle of incidence.
Show the same with the help of diagram.
Solution
In two
plane mirrors, the incident ray and reflected ray will always be parallel to
each other for any angle of incidence when the two
plane mirrors are placed at right angle to each other. The following diagram
illustrates this property.
Question: 29
Draw a ray
diagram showing the path of rays of light when it enters with oblique incidence:
(i) From
air into water
(ii) From
water into air
Solution
Long
answer Questions
Question: 30
Draw ray
diagrams showing the image formation by a concave mirror when an object is
placed:
(a) Between
pole and focus of the mirror
(b) Between
focus and centre of curvature of the mirror
(c) At
centre of curvature of the mirror
(d) A
little beyond centre of curvature of the mirror
(e) At
infinity
Solution
Question: 31
Draw ray
diagrams showing the image formation by a convex lens when an object is placed
(a) Between
optical centre and focus of the lens
(b) Between
focus and twice the focal length of the lens
(c) At
twice the focal length of the lens
(d) At
infinity
(e) At the
focus of the lens
Solution
Question: 32
Write laws
of refraction. Explain the same with the help of ray diagram, when a ray of
light passes through a rectangular glass slab.
Solution
The Laws of Refraction
(i) The incident ray,
the refracted ray and the normal to the interface of two transparent media at
the point of incidence, all lie in the same plane.
(ii) The ratio of sine
of angle of incidence to the sine of angle of refraction is a constant for a
given pair of media.
Let us consider the
refraction of light through the following glass slab.
Here AO is the
incident ray. OB is the refracted ray and BC is the emergent ray.
As
per the law of refraction, the incident
ray, the refracted ray and the normal to the interface of two transparent media
at the point of incidence (here O), all lie in the same plane.
The
ratio of sine of angle of incidence to the sine of angle of refraction is a
constant, for light of a given colour and for a given pair of media. This law
is also known as Snell’s law of refraction and is given as:
Question: 33
Draw ray
diagrams showing the image formation by a concave lens when an object is placed
(a) At the
focus of the lens
(b) Between
focus and twice the focal length of the lens
(c) Beyond
twice the focal length of the lens
Solution
Question: 34
Draw ray
diagrams showing the image formation by a convex mirror when an object is
placed
(a) At
infinity
(b) At
finite distance from the mirror
Solution
Question: 35
The image of a candle flame formed by a lens is obtained on a screen
placed on the other side of the lens. If the image is three times the size of
the flame and the distance between lens and image is 80 cm, at what distance
should the candle be placed from the lens? What is the nature of the image at a
distance of80 cm and the lens?
Solution
The magnification,
The object should be placed at a distance of 26.67 cm from the
lens.
The image is real and inverted. The lens is convex.
Question: 36
Size of image of an object by a mirror having a focal length of 20
cm is observed to be reduced to 1/3rd of its size. At what distance the object
has been placed from the mirror? What is the nature of the image and the
mirror?
Solution
As image could be real
or virtual, there are two possible situations.
Case I: In case of
concave mirror.
Case II: In case of a convex mirror
Therefore,
Question: 37
Define power of a lens. What is its unit? One student uses a lens
of focal length 50 cm and another of 50 cm. What is
the nature of the lens and its power used by each of them?
Solution
Power of a lens is the
degree of its convergence or divergence. It is the reciprocal of its focal
length in metres. Its unit is dioptre.
Case 1
The power of the lens in the first case,
It is a convex lens.
Case 2
The power of the lens in the second case,
It is a concave lens.
Question: 38
A student focussed the image of a candle flame on a white screen using
a convex lens. He noted down the position of the candle screen and the lens as
under:
Position of candle
Position of convex lens
Position of the screen
(i) What is the focal length of the convex lens?
(ii) Where will the image be formed if he shifts
the candle towards the lens at a position of 31.0 cm?
(iii) What will be the nature of the image
formed if he further shifts the candle towards the lens?
(iv) Draw a ray diagram to show the formation of
the image in case(iii) as said above.
Solution
(i)
Object distance,
Image distance
Therefore, (f is the focal length)
(ii) Object distance
on shifting is cm.
So, cm.
Since the object is placed at the
focus, the image will be formed at infinity.
(iii)
When the student further shifts the candle towards
the lens, the lens forms enlarged, virtual and erect image of the
candle.
(iv) The ray diagram
showing the formation of the image is given below.