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Q 1. With necessary figure, explain the refraction
of light through a rectangular glass slab.
Solution
Consider a
rectangular glass slab ABCD having parallel faces AB and CD as shown in above
figure. A ray of light EF in air is incident on the glass surface AB at point
O. As the ray EO enters from air (rarer medium) to glass (denser medium), the
ray bends towards normal and follows the path OO' inside the glass slab. At point
O', refraction takes place again. As the ray OO' enters from glass (denser
medium) to air (rarer medium), the ray bends away from normal and follow the
path O'H outside glass slab. Here, the ray EF is called incident ray, OO', the
refracted ray and O'H, the emergent ray. As per the laws of
refraction, and. The emergent ray O'H is parallel to the incident
ray EF. The perpendicular distance O'L between the original path of incident
ray and the emergent ray is called the lateral displacement.
Q 2. What is a spherical mirror? What are its two
types?
Solution
A highly polished
surface which is spherical or curved in shape is known
as spherical mirror. OR A mirror whose reflecting surface is a part of a
hollow sphere of glass is known as spherical mirror. A spherical mirror whose
reflecting surface is curved inwards is concave mirror. A spherical mirror
whose reflecting surface is curved outwards is convex mirror.
Q 3. Is mirror formula valid only for the plane
mirror?
Solution
No, the mirror formula
is valid for plane as well as for all spherical mirrors for all positions of
the object.
Q 4. (a) What is meant by magnification in the
context of image formation by mirror and lenses? (b) Power of a lens is + 5 D.
What is the focal length and nature of the lens?
Solution
(a) The ratio of the
size of the image to the size of the object is called as magnification, m =
hi/ho. (b) Lens is convex, f(m) = 1/P = 1/5 m = 20 cm
Q 5. Why is convex lens known as converging lens?
Solution
A convex lens is known
as converging lens because it converges parallel beam of light passing through
it.
Q 6. An object is kept at 70 cm in front of a
plane mirror. If the mirror is now moved 35 cm away from the object, how much
does the image shift from its previous position?
Solution
Initially, distance of
the object from the mirror = 70 cm. Therefore, image is formed at a distance 70
cm from the mirror, behind it. Thus, initial distance between the object and
image = 70 + 70 = 140 cm If the mirror is moved 35 cm away from the object, the
new distance of the object from the mirror = 70 + 35 = 105 cm. The new image is
now at a distance 105 cm from the mirror behind it. Thus, new distance of the
image from the object = 105 + 105 = 210 cm Taking the position of the object as
the reference point, the distance between the two positions of the image = New
distance of image from the object - Initial distance of the image from the
object = (210 - 140) cm = 70 cm Thus, the image shifts 70 cm away.
Q 7. What is refraction of light? How is it
related to refractive index?
Solution
The phenomenon of a
change in the direction of propagation of light when it passes obliquely from
one transparent medium to another is called refraction of light. For a given
angle of incidence, the extent of refraction of light is different in different
media. When the refractive index of the second medium is greater than the
refractive index of the first medium, more is the bending of the refracted ray
towards the normal. When the refractive index of the first medium is greater
than the refractive index of the second medium, lesser is the bending of the
ray of light away from the normal.
Q 8. Define power of a lens.
Solution
The power of a lens is
defined as the reciprocal of its focal length in metres.
Q 9. Give the three differences between real image
and virtual image.
Solution
Real image Virtual image
Image can be obtained on a screen. Image cannot be obtained on a screen.
Rays coming from an object actually meet at a point after
reflection. Rays coming from an object only appear to meet at a point
when produced backwards after reflection. It is inverted with respect to the
object. It is erect with respect to the object.
Q 10. Which lens has greater power, a convex lens
of focal length 20 cm or a concave lens of focal length 30 cm?
Solution
The convex lens of
smaller focal length (= 20 cm) has greater power since focal length is
reciprocal of power.
Q 11. What is difference between the light
reflected from a plane mirror and a cardboard?
Solution
The cardboard surface is
rough. Thus, irregular or diffuse reflection takes place. A parallel beam of
light incident on its surface is reflected in different directions. Mirror has
a smooth surface. A parallel beam of light incident on it gets reflected as
beam of parallel rays in one direction only. Thus, there is regular reflection
of light incident on a plane mirror.
Q 12. Find the focal length of a convex mirror
whose radius of curvature is 44 cm.
Solution
Q 13. Give three applications of plane mirror.
Solution
(i) Plane mirrors are
used in many optical instruments. (ii) Plane mirrors are used in periscopes and
kaleidoscopes. (iii) They are used in dressing rooms to see ourselves.
Q 14. When does Snell's law fail?
Solution
Snell's law fails when
the light rays are incident normally on the surface of a refracting medium. In
this case light passes undeviated from the surface, i.e. no refraction occurs.
Refractive index is n = sin i/sin r = 0/0 = 0. But, n is not zero. Thus,
Snell's law fails.
Q 15. How can you identify a plane mirror, a
concave mirror and a convex mirror without touching?
Solution
They can be identified
by observing the nature of image of our face. Plane mirror - produces virtual,
erect image of the same size as our face. Concave mirror - produces virtual,
erect and magnified image of our face Convex mirror - produces virtual, erect
and diminished image of our face
Q 16. Write any two uses of concave mirror.
Solution
Two uses of concave
mirror are as follows: (i) To collect solar radiations in solar heating
devices. (ii) As reflectors in torches, head lights, to get powerful beams of
light.
Q 17. To see a full length image of a tall
building, which type of mirror would you prefer to use?
Solution
Convex mirror. This is
because, it is curved outwards, which enables to view a large field area.
Q 18. Name the mirror which always produces an
erect and virtual image. How is the size of image related to the size of
object?
Solution
Convex mirror always
produces erect and virtual images. The images formed are diminished, i.e. the
size of the image is shorter than the size of the object.
Q 19. Why does a ray of light bend when it travels
from one medium into another?
Solution
When a ray of light
travels from one medium to another, its speed changes and this change in speed
of light causes a part of wave to travel slower than the other part. Thus,
bending of light takes place in refraction.
Q 20. Define reflection of light.
Solution
The phenomenon of
bouncing back of light or change in the path of light rays into the same medium
after striking a polished surface is called reflection of light.
Q 21. State the sign convention for lenses.
Solution
According to new
Cartesian sign convention for lenses i. All the distances are measured from the
optical centre of lens. ii. The distances measured in the same direction, as
that of incident light is taken as positive and against the direction of
incident light are taken as negative. iii. The distances measured upward and
perpendicular to the principle axis are taken as positive and downwards and
perpendicular to the principal axis is taken as negative.
Q 22. Do the laws of reflection change, when we
use a spherical mirror instead of a plane mirror?
Solution
No, the laws of
reflection do not change. They are applicable to spherical mirrors also.
Q 23. What are the units of refractive index?
Solution
Refractive index of a
medium has no units. This is because, it is the ratio of speeds, and ratio of
similar quantities is unitless.
Q 24. What will be the path of ray of light after
reflection, when it is incident on (a) focus and (b) centre of curvature of the
mirror?
Solution
(a) A ray of light that
passes through the focus, after reflection, becomes parallel to the principal
axis. (b) A ray of light that passes through the centre of curvature, after
reflection, retraces its original path.
Q 25. You are given two mirrors of equal size. One
is plane mirror and other is convex mirror. How will you identify them without
touching their surface?
Solution
By changing the position
of the object if the image size remained same and image position changes
equally as that of object distance, then the mirror is a plane mirror. But, by
changing the position of the object if the image size is changes and image is
always smaller than the size of the object, then the mirror is a convex mirror.
Q 26. A convex lens forms a real image 4 times
magnified at a distance of 60 cm from the lens. Calculate the focal length and
the power of the lens.
Solution
Magnification (m) = v/u
-4 = 60/u So, u = 60/-4 = -15cm 1/f = 1/v - 1/u = 1/60+1/15 = 5/60 Thus, f =
60/5 = 12 cm = 0.12 m P = 1/f = 1/0.12 = 8.33 D
Q 27. An object is placed perpendicular to the
principal axis of a convex lens having focal length 10 cm. The distance of an
object from the lens is 15 cm. Find the position of an image.
Solution
Q 28. Define the term refractive index of a
medium. Can it be less than 1?
Solution
The refractive index of
second medium with respect to first medium is defined as the ratio of the sine
of angle of incidence in the first medium to the sine of the angle of
refraction in the second medium. Refractive index of a medium is always greater
than 1 (it cannot be less than 1) because the speed of light in any medium is
always less than that in vacuum.
Q 29. The ray of light traveling in air enters
obliquely into water. Does the light ray bend towards the normal or away from
the normal? Why?
Solution
When a ray of light
travels from air into water obliquely, it bends towards the normal. This is
because water is optically denser than air. On entering into water, the speed
of light decreases and the light bends towards the normal.
Q 30. Give two characteristics of the image formed
by a concave lens.
Solution
Image formed by a
concave lens is virtual and diminished.
Q 31. (i) Describe an activity to find the
approximate value of focal length of a concave mirror. (ii) What happens to the
size of the image of an object when it is moved gradually away from a convex
mirror?
Solution
(i) Activity : Hold a
concave mirror in your hand and direct it towards the sun. Direct the light
reflected by the mirror on to a sheet of paper held close to the mirror. Move
the sheet back and forth slowly until a bright, sharp spot of light is seen on
the paper. The distance of this image from the position of the mirror gives the
approximate value of focal length of the mirror. (ii) Size of the image becomes
smaller
Q 32. What do you mean by a focal point of a
mirror?
Solution
Focal point is the
principal focus of the mirror where a parallel beam of light meets (or appear
to meet) after reflection from the mirror.
Q 33. Why is it difficult to shoot a fish swimming
in water?
Solution
When we peer through
water to observe fish swimming around the pond, they appear to be much closer
to the surface than they really are due to the refraction of light rays (as
they travel from water into the air). Light rays reflected from
fish are refracted at the surface of the water, but the eyes and brain trace
the light rays back into the water as if they had not been refracted but
traveled away from the fish in a straight line. This effect creates a 'virtual'
image of the fish that appears at a shallower depth. Hence, it is difficult to
shoot fish swimming in water.
Q 34. Define the term 'magnification'.
Solution
Magnification is defined
as the ratio of height of the image to the height of the object. It tells us
the amount by which the object is magnified with respect to the object.
Q 35. State the changes in the position, size and
nature of the image of an object when brought form infinity up to a convex
lens. Illustrate your answer by drawing diagrams.
Solution
(i) When the object
is situated at infinity, the position of image is at F2, it is very
much diminished in size and it is real and inverted. (ii) When the object
(AB) is situated beyond 2F1, the position of image (A'B') is between
F2 and 2F2, it is diminished in size and
real and inverted. (iii) When the object
(AB) is situated at 2F1, the position of image (A'B') is at 2F2,
it is of same size as the object and real and inverted. (v)When the object (AB) is
situated at F1, the position of image is at infinity; it is very
much magnified in size and real and inverted. (vi) When the object (AB) is
situated between lens and F1, the position of image (CD) is on the
same side, behind the object; it is magnified in size and virtual and
upright.
Solution
The perpendicular
distance of separation between the emergent ray and the incident ray is called
lateral displacement. It is seen that the emergent ray of light is parallel to
the incident ray but displaced from the path of the incident ray after
refraction through a glass slab.
Q 37. Define lateral inversion.
Solution
In the image formation
by plane mirror, the left side of the object appears to become the right side
of the image; and the right side appears to become the left side of image. This
change of sides of an object and its mirror image is known as lateral
inversion.
Q 38. (a) A concave mirror produces three times
enlarged image of an object placed at 10 cm in front of it. Calculate the
radius of curvature of the mirror. (b) What is a lens?
Solution
(a) The image formed in
front of the concave mirror is real, so m is negative, m = -3, u = -10 cm As m
= -v/u or -3 = -v/-10 v = -30 cm By mirror formula, 1/f = 1/v + 1/u = 1/-30 +
1/-10 = -4/30 or f = -30/4 = -7.5 cm Radius of curvature, R = 2f = 2 x
(-7.5) = 15cm (b) A lens is a piece of transparent glass bound by two spherical
surfaces.
Q 39. An object is placed perpendicular to the
principal axis of concave lens of focal length 30 cm. Find the position of an
image when the object is at a distance 20 cm from the lens.
Solution
Q 40. An object is placed at 4 cm distance in
front of a concave mirror of radius of curvature 24 cm. Find the position of
image by drawing. Is the image magnified?
Solution
The image is 6 cm behind
the mirror. Yes the image is magnified.
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