NCERT Solutions of Chapter 10“Light Reflection and Refraction”
Answer The Following Question.
1. Define
the principle focus of a concave mirror.
Answer:
Principal focus of a
concave mirror is the point on its principal axis, where light rays coming
parallel to principal axis actually converge after reflection from mirror.
2. The
radius of curvature of a spherical mirror is 20 cm. what is its focal length?
Answer:
Focal length (f)= R/2 =
20 cm/2 = 10 cm.
3. Name a
mirror that can give an erect and enlarged image of an object.
Answer:
Only a concave mirror can
give a erect and enlarged image of an object.
4. Why do
we prefer a convex mirror as a rear-view mirror in vehicles?
Answer:
We prefer a convex mirror
as a rear-view mirror in vehicles because a convex mirror gives an erect and
diminished image. As a result, convex mirror help the driver to have much wider
field view.
5. Find the
focal length of a convex mirror whose radius of curvature is 32 cm.
Answer:
Radius of curvature of
(R) = 32 cm
Focal length(f) = R/2 =
32/2 cm = 16 cm.
6. A concave mirror produces three times magnified real image of an object placed at 10 cm in front of it. Where is the image located?
Answer:
Distance of object from
concave mirror (u)= -10 cm.
Magnification (m) = -3
m = -v/u
v = -mu = -(3) x (-10) =
-30 cm.
7. A ray of light traveling in air enters obliquely into water. Does the light ray bend towards or away from the normal? Why?
Answer:
The light bends towards
the normal on entry into water. It is due to the fact that as compared to air,
the water is optically denser medium.
8. Light
enters from air to glass having refractive index 1.50. What is the speed of
light in glass? The speed of light in vacuum is 3×108 m/s.
Answer:
Speed of light in vacuum
(c) = 3 x 108 m/s.
Refractive index = c/v.
Speed of light in glass =
3 x 108 m/s/ 1.50
= 2 x 108 m/s
9. Find
out, from Table (10.3), the medium having highest optical density. Also, find
the medium with lowest optical density.
Answer:
As per table, diamond has
highest optical density (2.42). Medium with lowest optical density is air
(1.0003)
10. You are
given kerosene, turpentine and water. In which of these does the light travel
fastest? Use the information given in table 10.3
Answer:
As the refractive index
of water is least out of three substances, hence speed of light is maximum in
water. So, light travels fastest in water.
11. The
refractive index of diamond is 2.42. What is the meaning of this statement?
Answer:
It means that speed of
light in diamond is 2.42 times slower than speed of light in air.
12. Define
1 dioptre of power of lens.
Answer:
One dioptre of is defined
as the power of lens having a focal length of 1 m.
13. A
convex lens forms a real and inverted image of a needle at distance of 50 cm.
from it. Where is the needle placed in front of the convex lens if the image is
equal to the size of objects? Also, find the power of lens.
Answer:
Image distance (v) = +50
cm, hi = ho
hi/ho =
v/u
u = v x ho /
hi
= 50 x ho /
hi
= 50 cm.
Now,
u = -50 cm
v = + 50 cm.
f = ?
1/f = 1/v – 1/u
1/f = 1/50 + 1/50
f = + 25 cm. = 0.25 m
Power of lens (P) = 1/f
= 1/ 0.25 = + 4D.
14. Find
the power of a concave lens of focal length 2 m.
Answer:
Focal length of concave
lens = – 2 m.
P = 1/f = 1/ (-2m)
= -0.5 D
15. Which
one of the following materials cannot be used to make a lens?
(a) Water
(b) Glass
(c) Plastic
(d) Clay
Answer: (d)
Clay
16. The
image formed by a concave mirror is observed to be virtual, erect and larger
than the object. Where should be the position of the object?
(a) Between
the principal focus and the centre of curvature.
(b) At the
centre of curvature
(c) Beyond
the centre of curvature
(d) Between
the pole of the mirror and its principal focus.
Answer: (d)
Between the pole of the mirror and its principal focus.
17. Where
should an object be placed in front of convex lens to get a real image of the
size of the object?
(a) At the principal
focus of the lens.
(b) At twice the focal
length
(c) At infinity
(d) Between the optical
centre of the lens and its principal focus.
Answer: (b)
At twice the focal length.
18. A
spherical mirror and thin spherical lens have each of focal length of -15 cm.
the mirror and lens are likely to be
(a) Both concave
(b) Both convex
(c) The mirror is concave
and the lens is convex
(d) The mirror is convex
and lens is concave.
Answer: (a)
both concave.
19. No
matter how far you stand from a mirror, your image appears erect. The mirror is
likely to be
(a) Plane
(b) Concave
(c) Convex
(d) Either concave or
convex.
Answer: (d)
either plane or convex
20. Which
of the following lens would you prefer to use while reading small letters found
in a dictionary?
(a) A convex lens of
focal length 50 cm
(b) A concave lens of
focal length 50 cm
(c) A convex lens of
focal length 5 cm
(d) A concave lens of
focal length 5 cm
Answer: (c)
A convex lens of focal length 5 cm.
21. We wish
to obtain an erect image of an object, using a concave mirror of focal length
15 cm. What should be the range of distance of the object from mirror? What is
the nature of image? Is the image larger or smaller than the object? Draw a ray
diagram to show the image formation in this case.
Answer:
Object must be placed in front of concave mirror between its pole and principal focus at a distance less than 15 cm. The image formed will be virtual and erect. The size of the image is larger the object. The ray diagram is as follows:
22. Name
the type of mirror used in the following situations:
(a)
Headlights of a car
(b)
Side/rear-view mirror of a vehicle.
(c) Solar
furnace.
Support your answer with
reason.
Answer:
(a) Headlights of a car-
concave mirror to give parallel beam of light after reflection from concave
mirror.
(b) Side/rear-view mirror
of vehicle- convex mirror as it forms virtual erect and diminished image to
give wider view field.
(c) Solar furnace-
concave mirror to concentrate sunlight to produce heat in solar furnace.
23.
One-half of a convex lens is covered with a black paper. Will this lens produce
a complete image of the object? Verify your answers experimentally. Explain
your observations.
Answer:
When one-half of a convex
lens is covered with black paper, this lens produces a complete image of the
object. To prove it we perform an experiment:
When another-half of a convex lens is covered with a black paper
Take a concave mirror and
cover half part of its by using black paper. Place it vertically in a stand. On
one side of it place a burning candle. On opposite side of the lens fix a white
screen. Adjust the position of candle or screen till clear image of burning
candle is formed on the screen. We observe that the image is complete image of
the object. From the experimental observations, we find that image formation
does not depend upon the size of a lens. A similar lens can also form complete
image of an object placed in front of it. However, brightness of the image
decreases when some part of lens is blocked. It is because now lesser number of
rays pass through the lens.
24. An
object 5 cm in length is held 25 cm away from a converging lens of focal length
10 cm. Draw the ray diagram and find the position, size and the nature of the
image formed.
Answer:
Height of the Object, ho— 5 cm
Distance of the object fiom
converging lens, u — — 25 cm
Focal length of converging lens, f
— 10 cm
Using lens formula,
25. A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.
Answer:
f= -15 cm, v= -10 cm
1/v -1/u = 1/f
1/u = 1/15 – 1/10 = -1/30
u = -30 cm.
Ray diagram as follows:
26. An
object is placed at a distance of 10 cm from a convex mirror of focal length 15
cm. Find the position and nature of image.
Answer:
f = +15 cm, u = -10 cm.
1/f = 1/v +1/u
1/v = 1/15 +1/10
1/v = 5/30
v = + 30 cm.
The image is formed 6 cm
behind the mirror, it is a virtual and erect image.
27. The
magnification produced by a plane mirror is +1. What does this means?
Answer:
m= hi/h0=
v/u
Magnification produced by
a plane mirror is +1 which means that size of image formed is exactly equal to
size of object behind the mirror.
28. An
object 5.0 cm in length is placed at a distance of 20 cm in front of a convex
mirror of radius of curvature 30 cm. Find the position of the image, its nature
and size.
Answer:
Radius of curvature (R) =
30 cm
f = R/2 = 30/2 = 15 cm
u = –20 cm, h= 5 cm.
1/v +1/u = 1/f
1/v = 1/15+ 1/20 = 7/60
v = 60/7 = 8.6 cm.
image is virtual and
erect and formed behind the mirror.
hi/h0=
v/u
hi/5= 8.6/20
hi = 2.2
cm.
Size of image is 2.2 cm.
29. An
object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal
length 18 cm. At what distance from the mirror should a screen be placed, so
that a sharp focused image can be obtained? Find the size and the nature of the
image.
Answer:
u = – 27 cm, f = – 18 cm.
ho= 7.0 cm
1/v = 1/f- 1/u
1/v = -1/18 + 1/27 =
-1/54
V = – 54 cm.
Screen must be placed at
a distance of 54 cm from the mirror in front of it.
hi/h0=
v/u
hi/h0=
v/u
hi/7 = +54/-27
hi = -2 x
7 = -14 cm.
Thus, the image is of 14
cm length and is inverted image.
30. Find
the focal length of a lens of power -2.0 D. What type of lens is this?
Answer:
Power of lens (P) = -2.0
D
P = 1/f or f = 1/m
f = 1/-2.0 = -0.5 m.
(-ve) sign of focal
length means that the lens is concave lens.
31. A
doctor has prescribed a corrective lens of power +1.5 D. Find the focal length
of the lens. Is the prescribed lens diverging or converging?
Answer:
P = +1.5 D
f = 1/P = 1/+1.5 = 0.67
m.
As the power of lens is
(+ve), the lens is converging lens.
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