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Refraction of Light
1. What do you understand by refraction of light?
Ans. The phemonenon due to which a ray travelling in one optical medium, on
striking obliquely, the surface of another optical medium, deviates from its
path is called refraction of light.
2. What is the cause of refraction of light?
Ans. The speed and the wavelength of incident light changes on entering from one
medium to another medium. This results in the change in the direction of the
light.
3. Does reflection take place along with refraction?
Ans. Yes, a small amount of light gets reflected at the surface of separation of two
media.
4. State two laws of refraction.
Ans. (i) The incident ray, the refracted ray and the normal drawn at the point of
incidence on the surface of separation, lie in the same plane.
(ii) The ratio between the sine of angle of incidence (in rarer medium) to the sine
of angle of refraction (in the denser medium) is a constant quantity for given
two optical media and is called refractive index of denser medium with
respect to rarer medium.
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5. Define absolute refractive index of a medium.
Ans. The ratio between the velocity of light in vacuum and the velocity of light in a
given optical medium is called absolute refractive index of the optical medium.
6. For which colour of white light is the refractive index of a transparent medium
the least?
Ans. Refractive index of a transparent medium is the least in case of red light.
7. For which colour of white light is the refractive index of a transparent
medium
maximum?
Ans. Refractive index of a transparent medium is the maximum in case of violet light.
8. Name two factors on which refractive index of a medium depends.
Ans. (i) Refractive index of a medium depends upon its optical density.
(ii) Refractive index of a medium depends upon the wavelength of the incident
light.
9. Copy the diagram, and trace the course of rays through the glass block. In the
diagram, mark angle of incidence (i) and angle of refraction (r) at the first
interface. How the refractive index of glass is related to angle (i) and angle (r) ?
sin i
Ans. aμw =
sin r
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10. Diagram below, shows the path of ray of light through a glass slab. Copy the
diagram and mark the lateral displacement of incident ray. State two factors
which determine lateral displacement.
Ans. (i) Lateral displacement is directly proportional to thickness of the glass slab.
(ii) Lateral displacement is directly proportional to the angle of incidence.
11. Calculate the speed of light in water, when refractive index of water is 4/3 and
speed of light in vacuum is 3 × 108 ms–1.
Ans. aμw = Speed of light in vacuum ÷ Speed of light in water
∴ Speed of light in water
=
speed of light in vacuum
a
μw
3 ×108 ms –1 9
=
= × 108 ms–1
4/3
4
= 2.25 × 108 m s–1.
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12. Explain with the aid of labelled diagram, the appearance of stick partially
immersed in water.
Ans.
13. (i) Draw a properly labelled diagram to show that apparent depth of water in a
tank is less than the real depth.
(ii) How is the refractive index of water calculated from real and apparent depth?
Ans. (i) Labelled diagram is drawn below.
(ii) Refractive index of water =
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Real depth
Apparent depth
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14. Why do the faces of persons sitting around camp fire appear to shimmer ?
Explain.
Ans. The rays reflected from the face of person, sitting around the camp fire, suffer
refraction on passing through hot air. Thus, we see apparent image of person.
Since hot air is in motion, therefore its optical density changes, which in turn
shifts the image rapidly. This rapidly shifting image gives a shimmering effect.
15. Why does a fisherman aim at the tail of fish during spear fishing ? Explain.
Ans. Due to refraction, when the rays travel from water to air, the apparent image of
the fish is formed at a higher level and ahead of the actual position of fish. Thus,
when the fisherman aims at the tail of the apparent image of fish, his spear hits
the head of actual fish.
16. The speed of light in air is 3 × 108 ms–1. Calculate the speed of light in glass.
The refractive index of glass is 1.5.
Ans. Refractive index of glass =
Speed of light in air
Speed of light in glass
Speed of light in air
3×108ms –1
=
∴ Speed of light in glass =
Refractive index of glass
1.5
= 2 × 108ms–1.
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17. The speed of light in diamond is 125,000 kms–1. What is its refractive index?
(speed of light in air = 3 × 108 ms–1)
Ans. Refractive index of diamond
300,000 kms –1
Speed of light in air
=
=
= 2.4
Speed of light in diamond 125,000 kms –1
18. The refractive index of diamond with respect to air (a μ d ) = 2.4. Express the
refractive index of air with respect to diamond.
1
1
Ans. a μ d = a =
= 0.416
μ d 2.4
19. (a) Define prism.
(b) Define : (i) Refracting edge, (ii) Refracting angle, (iii) Angle of deviation,
(iv) Angle of emergence for a prism.
Ans. (a) A piece of glass or any other optical material, having two triangular and three
rectangular surfaces is called a prism.
(b) (i) The line along which two rectangular surfaces of a prism meet is called
refracting edge.
(ii) The angle subtended by the two rectangular surfaces of the prism at
the refracting edge is called refracting angle.
(iii) The angle between the incident ray and emergent ray, when a ray
suffers refraction through optical prism is called angle of deviation.
(iv) The angle between emergent ray and the normal is called angle of
emergence.
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20. (a) Draw a diagram to show the refraction of light of a single colour through
a prism. On the diagram show clearly : (i) incident ray, (ii) refracted ray,
(iii) emergent ray, (iv) angle of incidence (v) angle of emergence,
(vi) angle of deviation (vii) angle of prism.
(b) State the relation between ∠ i, ∠ e, ∠ A and ∠ δ.
Ans. (a)
(b) ∠ A + ∠ δ = ∠ i + ∠ e.
21. What do you understand by the term deviation produced by a prism? State the
factors on which angle of deviation depends.
Ans. The deviation (change in path) of the incident ray on emerging out of a prism is
called deviation. The angle of deviation depends upon :
(i) Angle of the prism.
(ii) Angle of the incidence.
(iii) Refractive index of the optical material.
(iv) Wavelength of incident light.
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22. Complete the following sentence by filling blank spaces
Angle of deviation is the angle which the .........(i)............ ray makes with the.
.........(ii)............. ray.
Ans. (i) emergent, (ii) incident.
23. What do you understand by the term angle of minimum deviation for a prism?
Ans. The minimum angle of deviation produced by a prism for some particular angle
of incidence is called angle of minimum deviation.
24. Draw a graph between the angle of incidence and angle of deviation for an
equilateral prism. On the graph clearly show the angle of minimum deviation.
Ans.
25. An equilateral prism is placed in minimum deviation position. If the angle of
incidence is increased, what happens to angle of deviation?
Ans. Angle of deviation will increase with the angle of incidence.
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26. An equilateral prism is placed in minimum deviation position. How the
following are related :
(i) angle of incidence ∠ i and angle of emergence ∠ e
(ii) angle of refraction at the two refracting faces i.e., ∠ r1 and ∠ r2.
Ans. In minimum deviation position.
(i) angle of incidence ∠ i = angle of emergence ∠ e.
(ii) angle of refraction ∠ r1 = angle of refraction ∠ r2.
27. A ray of light passes through an equilateral prism, such that refracted ray is
parallel to the base of prism. How are the angle of emergence ∠ e and angle of
incidence ∠ i related to each other.
Ans. Angle of incidence ∠ i = Angle of emergence ∠ e.
28. A yellow ray of light is incident on one of the refracting faces of an equilateral
prism, such that angle of deviation produced is minimum (36°) and the angle of
incidence is 48°.
(i) What is the angle of emergence?
(ii) If the angle of incidence is changed to : (a) 60° (b) 30°, state whether angle
of deviation will be equal to, less or more than 36°.
Ans. (i) In minimum deviation position.
Angle of emergence = Angle of incidence = 48°
(ii) (a) The angle of deviation will increase.
(b) The angle of deviation will increase.
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29. A ray of light is incident at an angle of 72° on the refracting face of an
equilateral prism. If the angle of deviation is 45°, calculate the angle of
emergence.
Ans. ∠ i + ∠ e = ∠ A + ∠ δ .
72° + ∠ e = 60º + 45º
∴ Angle of emergence ( ∠ e) = (105° – 72°) = 33°
30. The diagram below shows two identical prisms A and B placed with their
faces parallel to each other. A yellow ray of light PQ is incident at the face
of the prism A.
Complete the diagram to show the path of ray as it emerges out of prism B.
Ans.
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31. The diagram below shows an equilateral prism such that ray EF is incident
normally on the face AB. Answer the following questions :
(i) What is the angle of incidence on face AB?
(ii) What is the angle of refraction on face AB?
(iii) What is the angle of incidence on face AC?
(iv) Will the ray suffer minimum deviation by the prism?
Ans.(i) Angle of incidence is zero.
(ii) Angle of refraction is zero.
(iii) Angle of incidence on face AC is 60°.
(iv) The ray will not suffer minimum deviation.
32. (a) Explain the terms : (i) total internal reflection, (ii) critical angle.
(b) Write down a relation between refractive index and critical angle.
(c) Under what conditions total internal reflection takes place?
Ans.(a) (i) Total internal reflection : The phenomenon, due to which a ray of light
while travelling from denser to rarer medium, is reflected into denser
medium, at the interface of two media is called total internal reflection.
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(ii) Critical angle : The angle of incidence in denser medium, for which
angle of refraction in the rarer medium is 90°, is called critical angle.
(b) Refractive index =
a
or
μb =
1
sine of critical angle
1
sin C
(c) Conditions for total internal reflection :
(i) The rays of light must travel from denser to rarer medium.
(ii) The angle of incidence in denser medium, must be greater than critical
angle.
33. The diagrams given below show two glass prisms A and B. Two rays are shown
in diagram. Copy the diagrams and trace the path of rays, till they emerge out of
the prism.
Ans.
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34. A ray of light enters a glass slab ABCD as shown in the diagram. Copy the
diagram and complete the path of ray, till it emerges out of glass slab. Mark
the various angles wherever necessary. Critical angle for glass is 42°.
Ans.
35. Copy the diagram below and complete it to show the rays emerging out of
prism B.
State the principle used for completing your ray diagram.
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Ans. It is based on the principle of total internal reflection. The critical angle for
glass is 42°. Any ray of light travelling through glass at an angle more than
42°, suffers total internal reflection.
36. Figure given below shows a source of light S, placed at the bottom of a trough,
containing water.
(i) Copy the diagram and show the path of rays A, B and C.
(ii) Does ray A undergo refraction ?
(iii) Name the phenomenon exhibited by ray C.
(iv) State the conditions necessary for the phenomenon.
Ans. (i) The complete diagram is shown below.
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(ii) The ray A, does not undergo any refraction.
(iii) Ray C exhibits the phenomenon of total internal reflection.
(iv) (a) The ray of light must travel from the denser to the rarer medium.
(b) The angle of incidence in the denser medium should be greater than the
critical angle of medium.
37. Why is an empty test tube placed obliquely in water, appears filled with mercury,
when seen from above.
Ans. When the rays of light travelling through water strike the water-glass interface of
test tube at an angle which is more than critical angle for water, they suffer total
internal reflection. When these totally reflected rays reach the eye, then to the eye
they appear to come from the surface of test tube, which in turn appears filled with
mercury.
38. Why are the air bubbles rising up the fish tank appear silvery.
Ans. When the rays of light travelling through water, strike the water air interface of the
bubble at an angle which is greater than critical angle for water, they get totally
internally reflected. These reflected rays on reaching the eye appear to come from
the air bubble, which in turn appears silvery.
39. Why does a crack in a window pane appear silvery.
Ans. There is always present some amount of air in the crack. When the rays of light,
travelling through glass, strike the glass air interface at an angle, greater than
critical angle of glass, they are totally internally reflected. When these reflected
rays, reach eye then to the eye they appear to come from the crack, which in turn
appears silvery.
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40. Why do diamonds sparkle for some time, even in darkness.
Ans. The critical angle for diamond is very small i.e., 24°. Moreover, diamonds are
cut at sharp angles. Thus, the rays of light entering diamond suffer series of total
internal reflections. As the rays of light are trapped within the diamond, therefore
it sparkles.
41. State four differences between reflection and total internal reflection.
Ans.
Reflection
1. Smooth polished surface is
required for reflection.
2. It takes place for all angles of
incidence.
3. It takes place when the rays of light
travel from rarer denser medium
to an opaque medium.
4. Some amount of light is absorbed
by the reflecting surface.
Total Internal Reflection
1. No smooth polished surface is required for
total internal reflection.
2. It takes place only, when angle of incidence
is greater than critical angle.
3. It takes place when rays of light travel from
denser to rarer medium.
4. No light is absorbed by reflecting surface.
42. A coin is placed at a depth of 15 cm in a beaker containing water.
If the refractive index of water is 4/3 : Calculate the height through which the
image of the coin appear raised.
Ans. Real depth = 15 cm.
Refractive index of water = 4/3
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∴ Apparent depth =
Real depth
15 cm × 3
=
Refractive index
4
= 11.25 cm.
Height through which image of coin appears raised
= (15 – 11.25) cm = 3.75 cm.
43. A postage stamp placed under a glass slab, appears raised by 8 mm. If the
refractive index of glass is 1.5. Calculate the actual thickness of glass slab.
Ans. Let real thickness of glass slab = x
Apparent thickness of glass slab = (x – 8) mm.
Know, Refractive index of glass =
1.5 =
Real depth
Apparent depth
x
or 1.5 x – 12 mm = x
x – 8 mm
0.5 x = 12 mm or x =
12 mm
= 24 mm.
0.5
44. A convex lens and a concave lens are placed flat on a table. How will you
distinguish between the two, without touching ?
Ans. Try to pass a thin paper under each of the lens. The lens under which thin paper
passes is convex lens. The lens under which thin paper does not pass is concave
lens.
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45. In the diagram given below is shown an object AB, and its image I, formed by a
lens. Copy the diagram, draw the position of lens and mark its focal length.
Ans.
46. Show by a diagram the refraction of two incident rays parallel to the principal
axis passing through a convex lens by treating it as a combination of a glass
block and two triangular prisms.
Ans.
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47. Show by a diagram the refraction of at least three rays of light parallel to the
principal axis of a concave lens by treating it as a combination of a glass block
and two triangular prisms.
Ans.
48. Define the term principal axis of a lens.
Ans. An imaginary line joining the centres of curvature C1 and C2 of two spherical
surfaces of the lens is called principal axis.
49. Explain the term optical centre of a lens.
Ans. Optical centre is a point within the lens and on the principal axis, such that any
ray of light passing through it does not suffer any refraction, i.e., the ray
continues moving along the same path.
Or
It is a point within the lens where a line drawn through its aperture (diameter)
meets the principal axis.
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50. Define the term focal length of a lens.
Ans. It is a point on the principal axis of a lens, such that rays travelling parallel to
the principal axis, after refraction, either actually meet at this point or appear to
meet at this point.
51. Define the term focal length of a convex lens.
Ans. It is a point on the principal axis of a convex lens, such that rays travelling
parallel to the principal axis, after refraction, actually meet at this point.
52. Define the focal length of a concave lens.
Ans. It is a point on the principal axis of a concave lens, such that rays travelling
parallel to the principal axis, after refraction, appear to meet at this point.
53. What do you understand by the term first focal point of a convex lens. Illustrate
your answer by drawing a diagram. Also show first focal plane.
Ans.
It is a point on the principal axis of a convex lens, such that the rays starting
from it, after refraction, travel parallel to the principal axis. F1 is the first focal
point shown in the diagram.
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54. What do you understand by the term first principal focus (first focal point) of a
concave lens. Illustrate your answer by drawing a diagram. Also show first focal
plane.
Ans.
It is a point on the principal axis of a concave lens, such that rays which travel
along it, after refraction, travel parallel to the principal axis. F1 is the first principal
focus of the concave lens.
55. By drawing neat diagram, define second focal point (second principal focus) of a
convex lens. Also show the second focal plane.
Ans.
It is a point on the principal axis of a convex lens, such that the rays of light
coming parallel to principal axis, after refraction through lens, actually meet.
It is shown as F2 in the diagram.
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56. What do you understand by the term second focal point of a concave lens.
Illustrate your answer by drawing a neat diagram. Also show the second focal
plane.
Ans.
It is a point on the principal axis, such that the rays of light coming parallel to the
principal axis after refraction through concave lens, appear to diverge from this
point. It is shown as F2 in the diagram.
57. Draw diagram for the following situations for a single ray of light for convex lens
showing clearly the positions 2F1, F1, O, F2 and 2F2.
(a) When a ray of light is initially travelling parallel to principal axis.
(b) When a ray of light initially passes through F1.
(c) When a ray of light initially passes through optical centre.
Ans.(a)
(b)
(c)
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58. What do you understand by the term focal plane of a lens?
Ans. A vertical plane, passing through the principal focus of a lens is called its
focal plane.
59. Where do the following parallel beams of light meet on passing through a
convex lens when : (a) the beam is parallel to the principal axis. (b) the beam is
not parallel to the principal axis.
Ans. (a) The rays meet at the second principal focus of the convex lens.
(b) The rays meet in the second focal plane of the convex lens.
60. Copy and complete the diagrams given below. In the diagram indicate the
focal length of lens.
Ans.
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61. Draw a ray diagram to show how lens can be used as magnifying glass.
Ans. Magnifying glass
62. A convex lens forms a real, inverted and diminished image of an object.
Illustrate by drawing a neat diagram.
Ans.
63. An image is formed by a convex lens which is real, inverted and same size as
object. Draw a neat diagram to illustrate it.
Ans.
63. An image is formed by a convex lens which is real, inverted and enlarged.
Draw a neat diagram to illustrate.
Ans.
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65. An object is placed at the first principal focus of a convex lens. Draw a diagram
for the formation of image and state its characteristics.
Ans.
Characteristics of the Image :
1. Image is real.
2. Image is inverted.
3. Image is highly enlarged.
4. Image is formed at infinity on the other side of lens.
66. An object is placed anywhere between the infinity and optical centre of a concave
lens. Draw a diagram for the formation of imag.
Ans.
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67. What do you understand by the real image? State its characteristics.
Ans. When the rays of light strating from a point, after reflection or refraction actually
meet at some other point, then the image so formed, is called real image.
Characteristics :
1. It can be taken on the screen.
2. It is always inverted.
68. What is a virtual image? State its characteristics.
Ans. When the rays of light starting from a point, after reflection or refraction appear
to meet at some other point, then the image so formed is called virtual image.
Characteristics :
1. It can be taken on the screen.
2. It is always erect.
69. State the position of object, position of image; nature of image, when :
(i) Convex lens is used as burning grass.
(ii) Convex lens is used as objective lens of photographic camera.
(iii) Convex lens is used as an erecting lens in terrestrial telescope.
(iv) Convex lens is used in the cine projector.
(v) Convex lens is used in search light.
(vi) Convex lens is used in observing biological specimens.
Ans. (i) Object is at infinity
Image is formed at F2 on the other side of lens.
Image is real; inverted and almost diminished to a point.
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(ii) Object is in between infinity and 2F1.
Image is formed between F2 and 2F2.
Image is real; inverted and diminished.
(iii) Object is at 2F1.
Image is formed at 2F2.
Image is real; inverted and same size as object.
(iv) Object is between F1 and 2F1.
Image is between 2F2 and infinity.
Image is real; inverted and enlarged.
(v) Object is at F1.
Image is formed at infinity.
Image is real; inverted and highly enlarged.
(vi) Object is between F1 and O.
Image is formed on the same side of lens.
Image is virtual; erect and magnified.
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70. An object AB is placed on the principal axis of a convex lens as shown in the
figure below. Copy the diagram. Using three rays, starting from point A and the
properties of the points F1; O and F2, obtain the image formed by the lens.
[F1 and F2 are the first and second principal foci of lens]
Ans.
71. Does convex lens always forms real image? Give reason for your answer.
Ans. No. When the object is between its principal focus and optical centre, it always
forms a virtual, erect and enlarged image.
72. How can you determine the focal length of a convex lens using an optical pin
and a plane mirror?
Ans. Mount the convex lens and an object needle on their respective uprights. Place
the upright carrying the object needle in front of the lens on an optical bench.
Adjust a plane mirror at the back of the lens, such that its plane is vertical and
perpendicular to the axis of the lens.
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Move the needle forward and backward and adjust its position to get real and
inverted image of the needle formed by the lens, after reflection from the plane
mirror.
When the tip of the image in the plane mirror coincides with the tip of the
optical needle, i.e., when there is no parallax between the image and the
optical needle, the needle, is at the principal focus of the lens. The distance
between the optical needle and the lens gives the focal length of the lens.
The rays proceeding from the object at the principal focus after refraction
through the lens are rendered parallel. These parallel rays are reflected from
the plane mirror and after refraction through the lens, meet at the focus.
73. In the figure below a point source of light P, a convex lens L and a plane
mirror M are placed in such a way that light starting from P, returns back
to same point after refraction and reflection.
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1. What is the distance OP called?
2. To which point (left of P, on P or right of P) will the rays return, if the plane
mirror is made to touch convex lens?
Ans. (1) OP is called focal length.
(2) The rays will always return to point P.
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