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NCERT Solution for Class 10 Science Chapter 10 : Light Reflection and Refraction

Light Reflection and Refraction
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Chapter 10 of NCERT Class 10 Science is Light Reflection and Refraction, In this chapter, you will study about Light Reflection and Refraction with the help of laws of light reflection and refraction and identify the nature of light after striking on different shaped structures like curve, plane, etc. This chapter has 5 questions that will strengthen your concepts regarding the nature of light.

NCERT Solutions for Class 10 Science Chapter 10 by Swiflearn are by far the best and most reliable NCERT Solutions that you can find on the internet. These NCERT Solutions for Class 10 Science Chapter 10 are designed as per the CBSE Class 10th Science Syllabus. These NCERT Solutions will surely make your learning convenient & fun. Students can also Download FREE PDF of NCERT Solutions Class 10 Science Chapter 10.

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NCERT Solution for Class 10 Science Chapter 10 Light Reflection and Refraction PDF

Exercise: 10.1

Question 1:
Define the principal focus of a concave mirror.

Solution:
Light rays parallel to the principal axis of a concave mirror after reflecting from its surface converge at a specific point on its principal axis. This point of convergence of parallel light
rays is called 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?

Solution:
Given:
Radius of curvature of spherical mirror, R = 20 cm
Radius of curvature of a spherical mirror = 2 × Focal length (f)
R = 2f
f = R/2
= 20/2
= 10 cm
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.

Solution:
A concave mirror forms a virtual, erect, and enlarged image of an object placed between its pole and the principal focus. Neither plain nor convex mirror can make a virtual, erect, and
enlarged image.

Question 4:
Why do we prefer a convex mirror as a rear-view mirror in vehicles?

Solution 4:
When an object is placed in front of the convex mirror it gives a virtual, erect, and diminished image of the object. We need to see as many areas as possible behind the vehicle also we need all the images erect because of this criterion convex mirrors are preferred as a rear-view mirror in vehicles because and they give a wider field of view, this type of view allows the driver to see most of the traffic behind him.

Exercise: 10.2

Question 1:
Find the focal length of a convex mirror whose radius of curvature is 32
cm.

Solution 1:
Given:
Radius of curvature of the mirror = R = 32 cm
Radius of curvature of the mirror = 2 × Focal length (f)
R = 2f
f = R/2
= 32/2
= 16cm
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?

Solution 2:
Magnification produced by a spherical mirror is given by the relation,
m = Height of the image/ Height of the Object = – Image Distance/Object distance
m = h1/h0 = -v/u
Let’s assume the height of the object = h0 = h
Then, the height of the image, h1 = −3h (because the Image formed is real)
According to relation,
-3h/h = -v/u
v/u = 3
Given:
Object distance, u = −10 cm
v = 3 × (−10) = −30 cm
Image is formed at a distance of 30cm the mirror. Here, the negative sign indicates that an inverted image is formed.

Exercise: 10.3

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?

Solution 1:
Light refracts towards the normal while travelling from a rare medium to a dense medium. This is the reason why light bends towards normal while travelling from air into water.

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.

Solution 2:
Given: c = 300000000m/s
μ =1.5
Speed of light in glass = speed of light in vacuum / Reflective index of glass
1.5 = 3 x 108
vg
vg= 2 x 108 m/s
So, the speed of light in glass is 20000000 m/s.

Question 3:
Find out, from Table, the medium having highest optical density. Also find
the medium with lowest optical density.

Solution 3:
Highest optical density = Diamond with μ=2.42
Lowest optical density = Air with μ=1.0003
The optical density increases with an increase in the refractive index of the medium. A material which has the highest refractive index will have the highest optical density and viceversa.

Question 4:
You are given kerosene, turpentine and water. In which of these does the
light travel fastest? Use the information given in Table.

Solution 4:

Refer pdf.

Question 5:
The refractive index of diamond is 2.42. What is the meaning of this
statement?

Solution 5:

Refer pdf.

Exercise: 10.4

Question 1:
Define 1 dioptre of power of a lens.

Solution 1:
Dioptre is the SI unit of power of lens is denoted by the letter D. 1 dioptre can be defined as the power of a lens of focal length 1 metre.

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.

Solution 2:

Refer pdf.

Question 3:
Find the power of a concave lens of focal length 2 m.

Solution 3:
Focal length of concave lens, f = -2 m
Now, we know that,
P= 1/f
P=1/ (-2)
Hence, the power of the given lens is -0.5 D.
Here, negative sign arises due to the divergent nature of concave lens.

Exercise

Question 1:
Which one of the following materials cannot be used to make a lens?
(a) Water
(b) Glass
(c) Plastic
(d) Clay

Solution 1:
(d) Clay. A lens must allow the light rays to pass through it. Since clay does not have such property, it can’t be used in making 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 center of curvature
(b) At the center of curvature
(c) Beyond the center of curvature
(d) Between the pole of the mirror and its principal focus.

Solution:
(d) Between the pole of the mirror and its principal focus. The image formed is virtual, erect, and larger than the object, when an object is placed between the pole and principal focus of a concave mirror.

Question 3:
Where an object should 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.

Solution 3:
(b) At twice the focal length. The image is formed at the centre of curvature on the other side of the lens when an object is placed at the centre of curvature in front of a convex lens. It forms an image that 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

Solution 4:
(a) Both concave.
The primary focus of the concave lens in on the same side as the object while the same for a concave mirror is in front of the mirror. So we can see that both are negative because as per convention the focal length of a convex mirror and a concave lens are taken as negative. Hence, both the thin spherical lens and the spherical mirror 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

Solution 5:
(d)Both plane and convex mirror produce erect image for all object positions.

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

Solution 6:
(c)When the object is between F and O, a convex lens will make a magnified and erect image. Also, convex lenses having shorter focal length produce more magnification. Therefore, for reading small letters, using a convex lens of focal length 5 cm would be best.

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.

Solution 7:
When an object is placed between the principal focus (F) and pole (P), a concave mirror gives an erect image.. The image formed will be virtual, erect, and magnified in nature if the object is placed between the pole and principal focus of the concave mirror.

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.

Solution 8:
(a) Concave mirrors can produce a strong parallel beam of light when the light source is placed at their principal focus F. Hence, a concave mirror is used in the headlights of a car.
(b) A convex mirror is usually used in side /rear-view mirror of a vehicle. Convex mirrors are capable of giving a virtual, erect, and diminished image of the objects placed in front of it, as a result, a wider field of view is obtained. This wider field of view enables the driver to see most of the traffic behind him/her.
(c) In the solar furnace, concave mirrors are used. Concave mirrors are converging mirrors. Due to this property of concave mirrors they are used to construct solar furnaces. Concave mirrors converge the light incident on them at a single point of the focus known as the principal focus of the mirror. Hence, they produce a large amount of heat at their principal focus.

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.

Solution 9:
A full image but with lesser intensity is produced. We can understand these using two cases.
Case I: When the upper half of the lens is covered:
As the upper half of the lens is covered, the light rays coming from the object is refracted by the lower half of the lens only. After refraction, these rays meet at the other side of the lens to form the image of the given object.
Case II: When the lower half of the lens is covered:
As the lower half of the lens is covered, the light rays coming from the object is refracted by the upper half of the lens only. After refraction, these rays meet at the other side of the lens to form the image of the given object.
In both the cases some rays are blocked. But the other rays pass through the lens and form a complete image but the intensity of the image is less.

Question 10:
An object 5 cm in length is held 25 cm away from a converging lens of focal
length 10cm. Draw the ray diagram and find the position, size and the
nature of the image formed.

Solution 10:

Refer pdf.

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.

Solution 11:

Refer pdf.

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.

Solution 12:

Refer pdf.

Question 13:
The magnification produced by a plane mirror is +1. What does this mean?

Solution 13:

Refer pdf.

Question 14:
An object 5 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.

Solution 14:

Refer pdf.

Question 15:
An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of
focal length 18cm. 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.

Solution 15:

Refer pdf.

Question 16:
Find the focal length of a lens of power −2.0 D. What type of lens is this?

Solution 16:

Refer pdf.

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?

Solution 17:

Refer pdf.

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