NCERT solution of class 10 chapter llight reflection and refraction
Intext question
Intext question
Page no
168
Question
1:
Define
the principal focus of a concave mirror.
Answer
1:
Light
rays that are parallel to the principal axis of a concave mirror converge at a
specific
point
on its principal axis after reflecting from the mirror. This point is known as
the
principal
focus of the concave mirror.
Question
2:
The
radius of curvature of a spherical mirror is 20 cm. What is its focal length?
Answer
2:
Radius
of curvature, R = 20 cm
Radius
of curvature of a spherical mirror = 2 × Focal length (f)
Hence,
the focal length of the given spherical mirror is 10 cm.
Question
3:
Name
the mirror that can give an erect and enlarged image of an object.
Answer
3:
When
an object is placed between the pole and the principal focus of a concave
mirror,
the
image formed is virtual, erect, and enlarged.
Question
4:
Why
do we prefer a convex mirror as a rear-view mirror in vehicles?
Answer
4:
Convex
mirrors give a virtual, erect, and diminished image of the objects placed in
front
of
them. They are preferred as a rear-view mirror in vehicles because they give a
wider
field
of view, which allows the driver to see most of the traffic behind him
page
no.171
Question
1:
Find
the focal length of a convex mirror whose radius of curvature is 32 cm.
Answer
1:
Radius
of curvature, R = 32 cm
Radius
of curvature = 2 × Focal length (f)
R = 2f
Hence,
the focal length of the given convex mirror is 16 cm.
Question
2:
A
concave mirror produces three times magnified (enlarged) real image of object
placed
at
10 cm in front of it. Where is the image located?
Answer
2:
Magnification
produced by a spherical mirror is given by the relation,
Let
the height of the object, ho = h
Then,
height of the image, hI = −3h (Image formed is real)
Object
distance, u = −10 cm v = 3 × (−10) = −30 cm
Here,
the negative sign indicates that an inverted image is formed at a distance of
30 cm
in
front of the given concave mirror.
Page
no. 176
Question
1:
A ray of
light travelling in air enters obliquely into water. Does the light ray bend
towards
the normal
or away from the normal? Why?
Answer 1:
The light
ray bends towards the normal.
When a ray
of light travels from an optically rarer medium to an optically denser medium,
it gets
bent towards the normal. Since water is optically denser than air, a ray of
light
travelling
from air into the water will bend towards the normal.
Question
2:
Light
enters from air to glass having refractive index 1.50. What is the speed of
light in
the glass?
The speed of light in vacuum is 3 × 108 m s−1.
Answer 2:
Refractive
index of a medium nm is given by,
Speed of
light in vacuum, c = 3 × 108 m s−1
Refractive
index of glass, ng = 1.50
Speed of
light in the glass,
Question
3:
Find
out, from Table 10.3, the medium having highest optical density. Also find the
medium
with lowest optical density.
Answer
3:
Highest
optical density = Diamond
Lowest
optical density = Air
Optical
density of a medium is directly related with the refractive index of that
medium. A
medium
which has the highest refractive index will have the highest optical density
and
vice-versa.
It
can be observed from table 10.3 that diamond and air respectively have the
highest and
lowest
refractive index. Therefore, diamond has the highest optical density and air
has the
lowest
optical density.
Question
4:
You
are given kerosene, turpentine and water. In which of these does the light
travel
fastest?
Use the information given in Table.
Answer
4:
Speed
of light in a medium is given by the relation for refractive index (nm). The relation
is
given as
It
can be inferred from the relation that light will travel the slowest in the
material which
has
the highest refractive index and travel the fastest in the material which has
the lowest
refractive
index.
It
can be observed from table 10.3 that the refractive indices of kerosene,
turpentine, and
water
are 1.44, 1.47, and 1.33 respectively. Therefore, light travels the fastest in
water.
Question
5:
The
refractive index of diamond is 2.42. What is the meaning of this statement?
Answer
5:
Refractive
index of a medium nm is
related to the speed of light in that medium v by the
relation:
Where,
c is the speed of light in vacuum/air
The
refractive index of diamond is 2.42. This suggests that the speed of light in
diamond
will
reduce by a factor 2.42 compared to its speed in air.
Page
no. 184
Question
1:
Define
1 dioptre of power of a lens.
Answer
1:
Power
of lens is defined as the reciprocal of its focal length. If P is the
power of a lens of
focal
length F in metres, then
The
S.I. unit of power of a lens is Dioptre. It is denoted by D.
1
dioptre is defined as the power of a lens of focal length 1 metre.
1
D = 1 m−1
Question
2:
A
convex lens forms a real and inverted image of a needle at a 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
the
object? Also, find the power of the lens.
Answer
2:
When
an object is placed at the centre of curvature, 2F1, of a convex lens, its
image is
formed
at the centre of curvature, 2F2, on the other side of the lens. The image formed is
inverted
and of the same size as the object, as shown in the given figure.
It
is given that the image of the needle is formed at a distance of 50 cm from the
convex
lens.
Hence, the needle is placed in front of the lens at a distance of 50 cm.
Object
distance, u = −50 cm
Image
distance, v = 50 cm
Focal
length = f
According
to the lens formula,
Hence, the power of the given lens is +4 D.
Question 3:
Find the power of a concave lens of focal length 2 m.
Answer 3:
Focal length of concave lens, f = 2 m
Here, negative sign arises due to the divergent nature of concave
lens.
Hence, the power of the given concave
lens is −0.5 D.
Excercise
Question
1:
Which one
of the following materials cannot be used to make a lens?
(a) Water
(b) Glass
(c)
Plastic
(d) Clay
Answer 1:
(d) A lens
allows light to pass through it. Since clay does not show such property, it
cannot
be used to
make a lens.
Question
2:
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 2:
(d) When
an object is placed between the pole and principal focus of a concave mirror,
the image
formed is virtual, erect, and larger than the object.
Question
3:
Where
should an object be placed in front of a 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 3:
(b) When
an object is placed at the centre of curvature in front of a convex lens, its
image
is formed
at the centre of curvature on the other side of the lens. The image formed is
real,
inverted, and of the same size as the object.
Question
4:
A
spherical mirror and a thin spherical lens have each a focal length of −15 cm.
The mirror
and the
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, but the lens is concave
Answer 4:
By
convention, the focal length of a concave mirror and a concave lens are taken
as
negative.
Hence, both the spherical mirror and the thin spherical lens are concave in
nature.
Question
5:
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
plane or convex
Answer 5:
(d) A
convex mirror always gives a virtual and erect image of smaller size of the
object
placed in
front of it. Similarly, a plane mirror will always give a virtual and erect
image of
same size
as that of the object placed in front of it. Therefore, the given mirror could
be
either
plane or convex.
Question
6:
Which of
the following lenses 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 6:
(c) A
convex lens gives a magnified image of an object when it is placed between the
radius of
curvature and focal length. Also, magnification is more for convex lenses
having
shorter
focal length. Therefore, for reading small letters, a convex lens of focal
length 5
cm should
be used.
Question
7:
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 the mirror? What is the
nature
of the
image? Is the image larger or smaller than the object? Draw a ray diagram to
show
the image
formation in this case.
Answer 7:
Range of
object distance = 0 cm to15 cm
A concave
mirror gives an erect image when an object is placed between its pole (P) and
the
principal focus (F).
Hence, to
obtain an erect image of an object from a concave mirror of focal length 15 cm,
the object
must be placed anywhere between the pole and the focus. The image formed
will be
virtual, erect, and magnified in nature, as shown in the given figure.
Question
8:
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 8:
(a)
Concave
(b) Convex
(c)
Concave
Explanation
(a)
Concave mirror is used in the headlights of a car. This is because concave
mirrors can
produce
powerful parallel beam of light when the light source is placed at their
principal
focus.
(b) Convex
mirror is used in side/rear view mirror of a vehicle. Convex mirrors give a
virtual,
erect, and diminished image of the objects placed in front of it. Because of
this,
they have
a wide field of view. It enables the driver to see most of the traffic behind
him/her.
(c)
Concave mirrors are convergent mirrors. That is why they are used to construct
solar
furnaces.
Concave mirrors converge the light incident on them at a single point known as
principal
focus. Hence, they can be used to produce a large amount of heat at that point.
Question
9:
One-half
of a convex lens is covered with a black paper. Will this lens produce a
complete
image of
the object? Verify your answer experimentally. Explain your observations.
Answer 9:
The convex
lens will form complete image of an object, even if its one half is covered
with
black
paper. It can be understood by the following two cases.
Case I
When the
upper half of the lens is covered
In this
case, a ray of light coming from the object will be refracted by the lower half
of
the lens.
These rays meet at the other side of the lens to form the image of the given
object, as
shown in the following figure.
Case II
When the
lower half of the lens is covered
In this
case, a ray of light coming from the object is refracted by the upper half of
the
lens.
These rays meet at the other side of the lens to form the image of the given
object,
as shown
in the following figure.
Question
10:
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 10:
Object
distance, u = −25 cm
Object
height, ho = 5 cm
Focal
length, f = +10 cm
According
to the lens formula,
The
negative value of image height indicates that the image formed is inverted.
The
position, size, and nature of image are shown in the following ray diagram.
Question
11:
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 11:
Focal
length of concave lens (OF1), f = −15 cm
Image
distance, v = −10 cm
According
to the lens formula,
The
negative value of u indicates that the object is placed 30 cm in front of the
lens. This
is shown
in the following ray diagram.
Question
12:
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 the image.
Answer 12:
Focal
length of convex mirror, f = +15 cm
Object
distance, u = −10 cm
According
to the mirror formula,
The
positive value of magnification indicates that the image formed is virtual and
erect.
Question
13:
The
magnification produced by a plane mirror is +1. What does this mean?
Answer 13:
Magnification
produced by a mirror is given by the relation
The magnification
produced by a plane mirror is +1. It shows that the image formed by
the plane
mirror is of the same size as that of the object. The positive sign shows that
the
image
formed is virtual and erect.
Question
14:
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 14:
Object
distance, u = −20 cm
Object
height, h = 5 cm
Radius of
curvature, R = 30 cm
Radius of
curvature = 2 × Focal length
R = 2f f =
15cm
According
to the mirror formula,
The
positive value of image height indicates that the image formed is erect.
Therefore,
the image formed is virtual, erect, and smaller in size.
Question
15:
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 15:
Object
distance, u = −27 cm
Object
height, h = 7 cm
Focal
length, f = −18 cm
According
to the mirror formula,
The
negative value of magnification indicates that the image formed is real.
The
negative value of image height indicates that the image formed is inverted.
Question
16:
Find the
focal length of a lens of power −2.0 D. What type of lens is this?
Answer 16:
A concave
lens has a negative focal length. Hence, it is a concave lens.
Question
17:
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 17:
A convex
lens has a positive focal length. Hence, it is a convex lens or a converging
lens.
