5
LIGHT
✔ ) the most appropriate answer.
I. Tick (✔
1. Light causes the
(a) sensation of heat
(b) sensation of sound
(c) sensation of sight
(d) sensation of touch
2. Objects which emit light of their own are called
(a) non-luminous objects
(b) transparent objects
(c) translucent objects
(d) luminous objects
3. Glass allows light to pass through it because it is
(a) transparent (b) opaque
(c) luminous (d) non-luminous
4. When the sun, the earth and the moon come to lie in a straight line, it
causes
(a) earthquakes
(b) eclipses
(c) eruption of volcanoes
(d) none of these
5. A solar eclipse can occur only on a
(a) new moon day
(b) full moon day
(c) no moon day
(d) none of these
6. Images of an object can be formed by
(a) energy
(b) heat
(c) light
(d) gravity
7. Cellophane paper is an example of
(a) an opaque object
(b) a translucent object
(c) a transparent object
(d) a luminous object
8. Shadows are formed when light falls on
(a) a transparent object
(b) a luminous object
(c) an opaque object
(d) none of these
9. The completely dark position of a shadow is called
(a) penumbra (b) opaque
(c) umbra
(d) none of these
10. A solar eclipse is caused when the
(a) sun comes between the earth and the moon
(b) moon comes between the sun and the earth
(c) earth comes between the sun and the moon
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(d) none of these
11. The phenomenon in which light that strikes a surface is thrown back
into the same medium is called
(a) reflection (b) refraction (c) reflex action (d) regression
12. Which one of the following is the best reflector of light?
(a) wood
(b) pencil
(c) wall
(d) plane mirror
13. A ray of light falling on a mirror is a/an
(a) reflected ray (b) normal (c) incident ray (d) none of these
14. If the angle of incidence is 45°, then the angle of reflection will be
(a) 90°
(b) 22½°
(c) 45°
(d) 30°
15. The image formed by a plane mirror is
(a) erect
(b) inverted
(c) magnified (d) diminished
16. We can see an object from every direction due to
(a) regular refraction
(b) regular reflection
(c) irregular reflection
(d) none of these
17. A spherical mirror whose inner hollow surface is the reflecting surface
is a
(a) convex mirror
(b) concave mirror
(c) convex lens
(d) concave lens
18. The centre of a spherical mirror is called its
(a) aperture
(b) pole
(c) centre of curvature
(d) principal axis
19. The mirror used in a dressing table is a
(a) concave mirror
(b) convex mirror
(c) plane mirror
(d) none of these
20. A body which allows almost all the light to pass through it and does
not reflect any light is
(a) transparent body
(b) opaque body
(c) translucent body
(d) polished surface
Ans. 1. (c)
2. (d)
3. (a)
4. (b)
5. (a)
6. (c)
7. (b)
8. (c)
9. (c)
10. (b)
11. (a)
12. (d)
13. (c) 14. (c)
15. (a)
16. (c)
17. (b)
18. (c)
19. (c) 20. (a)
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II. Fill in the blanks.
1. We can see the objects only in the presence of _____________.
2. Moonlight is reflected _____________.
3. Tracing paper is a _____________ object.
4. Light travels in a _____________ line.
5. Rectilinear propagation of light causes the formation of __________
and ____________.
6. Eclipses are examples of the formation of _____________ in nature.
7. A lunar eclipse is formed on _____________ moon night.
8. The pin-hole camera demonstrates how light can form ___________
of object.
9. Charcoal is _____________ at room temperature but when it is
burnt it becomes _____________.
10. The primary natural source of light for us is _____________.
11. The image formed in the pin hole camera is _____________.
12. Irregular reflection is also called _____________.
13. Angle of incidence is the angle between _____________ and
_____________.
14. The image formed in a plane mirror is _____________.
15. The instrument used by sailors in submarines to look above the
water is _____________.
16. In a concave mirror, when the object is kept at the focus, the image
will be formed at the _____________.
17. Convex mirrors always form _____________ images.
18. Speed of light in vaccum is _____________.
19. Speed of light in water is _____________ than its speed in air.
20. A plane mirror _____________ all the light that falls on it.
21. The angle formed between the incident ray and the normal is called
_____________.
22. The incident ray, the reflected ray and the _____________ all lie in
the same plane.
23. The image that cannot be brought on a screen is called __________.
24. A _____________ reflection takes place when the light beam falls
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on a highly polished surface.
25. Once surface of a mirror is made opaque by silvering it followed by
a thin coat of _____________ paint.
26. The geometric centre of the hollow sphere of which a spherical
mirror is a part is called _____________.
27. A ray of light parallel to the principal axis of the mirror passes through
its _____________ after reflection.
28. The _____________ mirror is used for converging solar radiation
in solar cookers to generate heat.
29. The image formed by a plane mirror is of the _____________ size
as the object.
Ans. 1. light
2. sunlight
3. translucent
4. straight
5. lunar eclipse, solar eclipse
6. shadows
7. full moon day
8. image
9. non-luminous, luminous 10. sun
11. real
12. diffused reflection 13. incident ray, normal
14. virtual
15. periscope
16. infinity
8
19. 225,000 km/s
17. virtual
18. 3 × 10 m/s
20. reflects
21. angle of incidence 22. normal
23. virtual image
24. regular
25. silver
26. centre of curvature 27. focus
28. concave
29. same
III. Fill in the blank spaces by choosing correct words from the list
given below.
List : translucent, normal, principal focus, behind, virtual image, radius
of curvature, diminished, new moon, visible, rectilinear and invisible.
1. Light is an _________ energy which causes in us sensation of vision.
2. Ground glass is a _____________ body.
3. Solar eclipse takes place on _____________ day.
4. Pin hole camera is based on _____________ propagation of light.
5. The angle between reflected ray and the _____________ is called
angle of reflection.
6. The image formed in a plane mirror is formed as far _____________
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7.
8.
9.
10.
Ans.
the mirror as the object is in front of it.
The image which can be only seen by the eye but cannot be taken
on screen is called _____________.
The distance between the pole and centre of curvature of a spherical
mirror is called _____________.
Any ray of light on passing through _____________ of a concave
mirror after reflection, travels parallel to principal axis.
The image formed in case of convex mirror is always ___________.
1. invisible
2. translucent
3. new moon
4. rectilinear and invisible
5. normal
6. behind
7. virtual image
8. radius of curvature
9. principal focus
10. diminished
IV (A). Match the following.
1. luminous object
(a) water
2. non-luminous object
(b) shadows
3. transparent object
(c) ground glass
4. translucent object
(d) fluorescent tube
5. eclipses
(e) spoon
Ans. 1. (d), 2. (e), 3. (a), 4. (c), 5. (b)
IV (B). Match the following.
1. kaleidoscope
(a) convex mirror
2. searchlight
(b) object placed at focus
3. rear-view mirrors
(c) regular reflection
4. reflection by mirrors
(d) concave mirror
5. reflection by wood
(e) plane mirror
6. image formed at infinity
(f) irregular reflection
Ans. 1. (e), 2. (d), 3.(a), 4. (c), 5. (f), 6. (b)
V. Find the odd-one out. Give reasons for your choice.
1. Electric bulb, sun, moon, fire
Ans. Moon. Electric bulb, sun and fire are luminous objects while moon
is non-luminous object.
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2. Wood, brick, glass, cement.
Ans. Glass. Wood, brick and cement are opaque objects while glass is
transparent object.
3. Shadows, eclipses, expansion, images
Ans. Expansion. Shadows, eclipses and images, all are the phenomena
due to light while expansion is the phenomenon due to heat.
4. Umbra, penumbra, shadows, image
Ans. Image. Umbra and penumbra are the regions of shadows.
5. Wood, steel, magnet, mirror, graphite
Ans. Mirror. Wood, steel, magnet and graphite all are opaque bodies
and show irregular reflection while mirror is a highly polished
surface and shows regular reflection.
6. Real image, virtual image, inverted image, diminished image
Ans. Diminished image. Because it is related to the size.
7. Shaving mirror, rear-view mirror, street lamps, vigilance mirrors
Ans. Rear-view mirror. Shaving mirror, street lamps and vigilance
mirrors all uses concave mirrors while convex mirror is used as
rear-view mirror.
VI. Statements given below are incorrect. Write the correct statements :
1. A dark patch formed behind an opaque body when the opaque
body is placed in the path of light is called an image.
Ans. A dark patch formed behind an opaque body when the opaque body
is placed in the path of light is called a shadow.
2. When the rays of light starting from a point travel in various
directions, then the collection of such rays is called convergent beam
of light.
Ans. When the rays of light starting from a point travel in various directions,
then the collection of such rays is called divergent beam of light.
3. Pin hole camera can take pictures of moving objects.
Ans. Pin hole camera can take pictures of rest objects.
4. A region of total darkness is called penumbra.
Ans. A region of total darkness is called umbra.
5. Solar eclipse is a frequent phenomenon.
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Ans. Lunar eclipse is a frequent phenomenon.
6. A ray of light which bounces off the surface of a mirror is called
incident ray.
Ans. A ray of light which bounces off the surface of a mirror is called
reflected ray.
7. Image formed in a plane mirror is always inverted.
Ans. Image formed in a plane mirror is always erect.
8. Reflecting periscope is used by barbers to show the back of head.
Ans. Plane mirror is used by barbers to show the back of head.
9. The image formed in convex mirror is always erect and enlarged.
Ans. The imaged formed in convex mirror is always erect and small in
size.
VII. Write true or false for each statement.
1. A hot filament of an electric bulb gives out light.
2. Moon has no light of its own.
3. The sun will not be visible during a lunar eclipse.
4. Opaque objects do not allow light to pass through them.
5. The shadow formed by an object is of the same size as the object.
6. A concave mirror is used for obtaining a real and magnified
image.
7. A convex mirror is used as reflector in the car head lights.
8. Glass is a transparent body.
9. Gold is a translucent metal.
10. Virtual image is always erect.
11. A convex mirror always forms a real image.
12. A concave mirror is always used as a street light reflector.
13. A periscope does not work in foggy weather.
14. Solar eclipse is a very common phenomenon.
15. A real image is always inverted.
16. A glass allows almost all the light to pass through it.
17. Real image is formed by a plane mirror.
18. Regular reflection is produced when light falls on the walls.
19. In spherical mirror, the reflecting surface is curved.
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20. A ray of light passing through the centre of curvature of the mirror is
reflected back along the same path.
21. The angle of incidence is twice the angle of reflection.
Ans. 1. True
2. True
3. False
4. True
5. False
6. True
7. False
8. True
9. False
10. True
11. False 12. True
13. True
14. False
15. True
16. True
17. False
18. False
19. True
20. True
21. False
VIII. Differentiate between the following.
1. Luminous objects and non-luminous objects
Ans.
(a)
(b)
(c)
(d)
Luminous objects
The objects which emit light
energy by themselves are
called luminous objects.
They cause the sensation of
light.
Luminous objects are seen
on their own.
The Sun, the candle, the
electric bulb are some
luminous objects.
Non-luminous objects
The objects which do not emits
light energy themselves are
called non-luminous objects.
They do not cause the
sensation of light.
Non-luminous objects are
seen due to luminous objects.
The moon, the chair, the
plants are some non-luminous
objects.
2. Transparent objects and opaque objects
Ans.
Transparent objects
(a) These objects allow light to
pass through them.
(b) These objects do not form
shadow in presence of light.
(c) Glass and water are some
transparent objects.
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Opaque objects
These objects never allow
light to pass through them.
These objects form shadows
in presence of light.
Wood, stone and brick are
some opaque objects.
Question Bank
3. Umbra and Penumbra
Ans.
Umbra
(a) It is the region of total
darkness in the shadow.
(b) The ray of light enters in
umbra region.
(c) Umbra is surrounded by
penumbra.
Penumbra
It is the region of partial
darkness in the shadow.
Some rays of light enter in
penumbra region.
Penumbra surrounds the
umbra.
4. Regular reflection and irregular reflection
Ans.
Regular reflection
(a) In regular reflection, all the
light that falls on the
refelecting surface is reflected
in a definite direction.
(b) It takes place on a smooth
highly polished surface.
(c) This property of light is used
in search light.
Irregular reflection
In irregular reflection, all the
light that falls on the reflecting
surface is not reflected in a
definite direction.
It takes place on a rough and
uneven surface.
Due to irregular reflection, we
are able to see things around us.
5. Plane mirror and spherical mirror
Ans.
Plane mirror
Spherical mirror
(a) It is highly polished plane It is the mirror in which curved
surface is reflecting surface.
surface.
It is of two types : concave
(b) It is of single type.
mirror and convex mirror.
(c) Plane mirror always form Spherical mirror do not form
equal-sized image always.
equal-sized image.
(d) Plane mirror has no centre Spherical mirror has centre of
curvature.
of curvature.
6. Real image and virtual image
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Ans.
Real image
(a) Real images are formed by
the rays of light after
reflection or refraction meet
at a point.
(b) It can be taken on screen.
(c) It is always inverted.
Virtual image
Virtual images are formed by
the rays of light after reflection
or refraction appear to meet at
a point.
It cannot be taken on screen.
It is always erect.
IX. Give reasons for the following.
1. Looking glasses are plane mirrors.
Ans. Looking glasses are the plane mirrors because the image formed by
a plane mirror is always erect and of same-size as that of the object.
2. A mirror reflects sunlight to give a bright patch of light on the wall,
while the lid of a tin gives a less bright patch.
Ans. A mirror reflects sunlight to give a bright patch of light on the wall
because mirror is a polished surface and give regular reflection while
the lid of a tin being uneven and rough gives a less bright patch due
to diffused reflection.
3. A person uses a concave mirror when applying make-up.
Ans. Concave mirrors are used as shaving mirrors or as make-up mirrors
because it gives enlarged and erect image of the face.
X. Write a short note on: (i) Bioluminescence (ii) Eclipses (iii) Translucent
object
Ans. (i) Bioluminescence. In nature, there are a few plants and animals
that emit light as seen in the glow of a firefly’s tail. The emission of
light by an organisms called bioluminescence. Bioluminescence results
from a chemical reaction in which chemical energy is converted into
light energy. In this process, a little amount of heat energies given
off. Due to this reason, this emission is called cold light.
(ii) Eclipses. We know that the earth and the moon both are opaque
and non-luminous bodies and the sun is luminous body. The earth and
the moon both cast their shadows leading to the phenomena of eclipses.
There are two types of eclipses: the lunar and the solar eclipses.
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(iii) Translucent objects. The objects that allow only a part of
light falling on them to pass through is called translucent objects. If
we see an electric bulb through these objects we will not be able to
see it clearly. It looks hazy. Oil paper, tracing paper and ground
glass are some examples of translucent object.
XI. Answer the following questions.
1. Name one natural source and one artificial source of light.
Ans. Sun is the natural source of light and electric bulb is the artificial
source of light.
2. Name two objects which are non-luminous at room temperature but
can be made luminous by heating.
Ans. Charcoal and kerosene oil both are non-luminous at room temperature
but can be made luminous by heating.
3. If we hold a burning candle on the other side of the following object,
state whether we would be able to see the flame clearly, hazy or not
able to see the flame at all. (a) cardboard (b) stone (c) polythene sheet
(d) cellophane paper (e) paraffin wax (f) glass (g) ground glass
Ans. We cannot see a burning candle clearly through cardboard and stone.
Through polythene sheet, cellophane paper, paraffin wax, and ground
glass, we will not be able to see the flame clearly. It looks hazy.
Glass is a transparent body and allows light to pass through it.
Hence, we can see the burning candle clearly.
4. List three effects of rectilinear propagation of light.
Ans. (i) formation of shadow.
(ii) formation of day and night.
(iii) Headlights of a car shows the rectilinear propagation of light.
5. List three characteristics of a shadow formed.
Ans. When an object is placed between a point light source and the screen,
the shadow formed is:
(i) uniformly dark
(ii) of the same shape as the object
(iii) sharp at the edge.
6. List two factors on which the size and nature of shadow depends.
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Ans. The size and nature of shadow depend on the source of light and
the position of the object with respect to the source and the screen.
7. Explain why we cannot see the shadow of a kite flying very high in
the air.
Ans. We cannot see the shadow of a kite flying high up in the air because
umbra is absent and penumbra is too large and too faint to be visible.
8. Give an experiment to demonstrate that light travels in a straight
line.
Ans. (a) Take three cardboard squares of equal size. Locate the centre
of each pieces of cardboard by drawing the diagonals.
(b) With the help of a nail, make a hole at the centre of each cardboard.
(c) Now fix the three cardboards on plasticine or on stands so
that they remain upright.
(d) Arrange the three cardboards A, B and C, one behind the
other such that their centres are in the same horizontal line.
You may pass a knitting needle through the holes to confirm if
they are in a straight line.
(e) Now place a burning candle in front of the board C and look
through the pinhole in board A. The flame will be clearly visible.
This shows that light travels in a straight line. Now, move
board B slightly and again look through the pinhole in board A
as shown in the figure.
You will not be able to see the flame. This shows that light does not
travel in a zig-zag way.
Rectilinear propagation of light
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9.
Ans.
10.
Ans.
11.
Ans.
12.
Ans.
13.
Ans.
By this experiment we can conclude that light travels in a straight
line. This property of light is called rectilinear propagation of light.
When does a lunar eclipse take place ?
When the earth comes in between the sun and the moon, the light
reaching the moon from the sun is obstructed by the earth. This
makes the moon invisible to the observer on the earth and thus the
lunar eclipse takes place.
State four characteristics of an image formed in plane mirror.
(i) Image is erect and of equal size.
(ii) Image is virtual, it means the image cannot be taken on screen.
(iii) The distance of image from the mirror is same as the distance
between object and mirror.
(iv) Image is laterally inverted. It means right hand side of the object
appears as left hand side of the image and vice versa.
State laws of reflection.
When the rays of light are reflected back by the second optical
medium into the first optical medium with the change in angle then
this phenomenon is called reflection. In this phenomenon.
(i) The incident ray, the reflected ray and the normal all lie in the
same plane at the point of incidence.
(ii) the angle of incidence is always equal to the angle of reflection
∠i = ∠r
State four uses of plane mirror.
(i) As a looking glass.
(ii) In solar cookers for reflecting the rays of sun.
(iii) For making reflecting periscope.
(iv) For making toys like Kaleidoscope.
Define the following : (a) Principal focus (b) focal length (c) centre
of curvature and (d) radius of curvature.
(a) Principal focus: The point on the principal axis where a beam
of light, travelling parallel to principal axis after reflection meet
(in case of cancave mirror) or appears to come (in case of
convex mirror) is called principal focus.
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(b) Focal length: The distance between the pole (P) and focus
(F) is called the focal length.
(c) Centre of curvature : The geometric centre of hollow sphere
of which the spherical mirror is a part is called the centre of
curvature of the spherical mirror. It is denoted by C.
(d) Radius of curvature : The radius of the hollow sphere of
which the spherical mirror is a part is called the radius of
curvature of the spherical mirror.
14. Draw neat diagrams and state characteristics of image formed when
(a) an object is placed between the principal focus and the centre
of curvature of concave mirror.
(b) an object is placed between the principal focus and the pole
of concave mirror.
(c) an object is anywhere between the pole and infinity of convex
mirror.
Ans. (a) When an object is placed between principal focus and the
centre of curvature of concave mirror then the image formed
will be real, inverted, enlarged and formed beyond the centre
of curvature but not at infinity.
(b) When an object is placed between the focus and the pole of
concave mirror than the image formed will be virtual, erect,
magnified and formed behind the mirror.
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(c) The image formed by the convex mirror is erect but smaller in
size and formed behind the mirror when the object is anywhere
between the pole and infinity of convex mirror.
15. State two uses of (a) concave mirror (b) convex mirror.
Ans. (a) Uses of concave mirror:
(i) as a shaving mirror.
(ii) as a reflector in the headlights of automobiles.
(b) Uses of convex mirror:
(i) as a rear view mirror.
(ii) as a street light reflector.
16. State two differences between a real image and a virtual image.
Ans. (i) Real image can be obtained on screen whereas virtual image
cannot be obtained on screen.
(ii) Real image is always inverted whereas virtual image is always
erect.
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17. You are provided with(a) plane mirror (b) concave mirror (c)convex
mirror. How will you distinguish them without touching?
Ans. We can distinguish different types of mirror on the basis of image
formation of an object.
(a) A plane mirror always forms an erect and equal-sized image of
an object.
(b) A concave mirror forms an inverted and enlarged image of an
object.
(c) A convex mirror always forms an erect and small-sized image
of an object.
18. What is a concave mirror?
Ans. A spherical mirror, which is polished from outer curved side. Such
that its reflecting surface is towards the hollow side is called concave
mirror.
19. Where should you keep the object to get a real, enlarged image with
a concave mirror?
Ans. When we keep the object between principal focus (F) and centre of
curvature (C) of concave mirror then the image will be real, enlarged
and formed beyond the centre of curvature but not at infinity.
20. What property of concave mirrors is used in making of search light?
Ans. If a point source of light is placed at the focus of a concave mirror,
the rays of light starting from focus, after reflection from the mirror
are rendered parallel to the principal axis. This property of concave
mirror is used in the making of search light.
21. What types of mirror is used as a rear-view mirror in vehicles and why?
Ans. Convex mirror is used as a rear-view mirror in vehicle.
The images formed by convex mirror are erect and smaller in size
and formed behind the mirror. Because of this property, convex
mirrors are used by bus or car drivers to get a wide view of whole
traffic coming from behind.
22. Monica, 145 cm tall stood at a distance of 2 metres away from a
plane mirror.
(a) What is the distance between Monika and her image?
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Ans.
(b) What will be the height of Monika’s image?
(c) What will be the distance between Monika and her image if
she walks a distance of 1 m towards the mirror?
(d) How much further she has to walk to be only 1 m from her image.
(a) We know that in a plane mirror, the image formed on the same
distance as the distance between object and mirror. So, the
distance between Monika and her image
= 2 + 2 = 4 metres.
(b) The height of Monika will be same i.e., 145 cm.
(c) If Monika walks a distance of 1 metre towards the mirror then
there will be short of two metres in total distance.
i.e.,
4 – 2 = 2 metres
(d) She has to walk only ½ metre further (total 1½ m) to be only
1 metre from her image.
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