# NCERT Solutions Class 10 for Science Chapter 10 Light, Reflection and Refraction

NCERT Solutions Class 10 for **Science Chapter 10 Light, Reflection and Refraction** : In this post, we will share with you all the detailed **NCERT Solutions of Class 10 Science Chapter 10 Light, Reflection and Refraction**. This will contain both in-text and back-exercise questions for Science and Social Science, and all exercise questions for Mathematics. For all school and board level examinations, doing all the NCERT Questions is a must.

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# Class 10 Science Chapter 10 Light,Reflection and Refraction

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**In-Text Questions** (Page 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 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?

**Answer 2:**

Radius of curvature (R) = 20 cm

Radius of curvature of the spherical mirror = 2 × Focal length (f)

R = 2f

f= R/2 = 20 / 2 = 10

Therefore, the focal length of the spherical mirror is 10 cm.

**Question 3:**

Name the mirror that can give an erect and enlarged image of an object.

**Answer 3:**

The mirror that can give an erect and enlarged image of an object is Concave Mirror.

**Question 4:**

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

**Answer 4:**

Convex mirror is preferred as a rear-view mirror in cars and vehicles as it gives a wider field of view, which helps the driver to see most of the traffic behind him. Convex mirrors always form an erect, virtual, and diminished image of the objects placed in front of it.

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## In-Text Questions (Page 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

f = R/2 = 32/2 = 16

Therefore, the focal length of the given convex mirror is 16 cm.

**Question 2:**

A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located?

**Answer 2:**

Because the image is real, so magnification m must be negative.

Thus the image is located at a distance of 30 cm from the mirror on the object side of the mirror.

## In-Text Questions (Page 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 because the ray of light goes from a rarer medium to a denser medium.

**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 x 108 ms-1

**Answer 2:**

Refractive index of a medium (nm) = Speed of light in vacuum/Speed of light in the medium

Speed of light in vacuum (c) = 3 × 108 m/s

Refractive index of glass (ng) = 1.50

Speed of light in the glass (v) = Speed of light in vacuum/ Refractive index of glass

= c/ng

=3 × 108/1.50 = 2x 108 ms-1.

**Question 3:**

Find out, from Table 10.3, the medium having highest optical density. Also find the medium with lowest optical density.

**Answer 3:**

From table 10.3, diamond has highest refractive index (= 2.42), so it has highest optical density.

Air has lowest refractive index (= 1.0003),

so it has 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 10.3.

**Answer 4:**

For kerosene, n = 1.44

For turpentine, n = 1.47

For water, n = 1.33

Because water has the lowest refractive index, therefore light travels fastest in this optically rarer medium than kerosene and turpentine oil.

**Question 5:**

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

**Answer 5:**

By saying that the refractive index of diamond is 2.42, we mean that the speed of light in diamond is lower by a factor of 2.42 relative to that in vacuum.

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## In-Text Questions (Page 184)

**Question 1:**

Define 1 dioptre of power of a lens.

**Answer 1:**

One dioptre is the power of a lens whose focal length is 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. , Sol. Here, u — +50 cm .

**Answer 2:**

Here ν = +50cm

Because the real image is of the same size as the object,|

**Question 3:**

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

**Answer 3:**

Focal length of concave lens (f) = 2 m

Power of lens (P) = 1/f = 1/ (-2) = -0.5D

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## Exercise Questions (Page 185-186)

**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) Clay cannot be used to make a lens because if the lens is made up of clay the light rays cannot pass through it

**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) The position of the object should be between the pole of the mirror and its principal focus.

**Queston 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) The object should be placed at twice the focal length

**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:**

(a) Both are likely to be concave.

**Question 5:**

No matter how far you stand from 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) The mirrors are likely to be either plane or convex

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**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 of focal length 5 cm.

**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 the distance of the object = 0 to 15 cm from the pole of the mirror.

Nature of the image = virtual, erect, and larger than the object.

**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 mirrors are used as reflectors in headlights of cars. When a bulb is located at the focus of the concave mirror, the light rays after reflection from the mirror travel over a large distance as a parallel beam of high intensity.

(b) A convex mirror is used as a side/rear-view mirror of a vehicle because

i.A convex mirror always forms an erect, virtual and diminished image of an object placed anywhere in front it.

ii.A convex mirror has a wider field of view than a plane mirror of the same size.

(c) Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.

**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:**

Yes, it will produce a complete image of the object, as shown in the figure. This can be verified experimentally by observing the image of a distant object like a tree on a screen when the lower half of the lens is covered with a black paper. However, the intensity or brightness of the image will reduce.

**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:**

Height of the Object, h0 = 5 cm

Distance of the object from converging lens, u = -25 cm

Focal length of a converging lens, f = 10 cm

Using lens formula,

Thus, the image is inverted and formed at a distance of 16.7 cm behind the lens and measures 3.3 cm. The ray diagram is shown below.

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**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:**

Object distance, u = -10 cm, Focal length, f = +15 cm, Image distance, ν = ?

Thus, image distance, ν = + 6 cm

Because ν is +ve, so a virtual image is formed at a distance of 6 cm behind the mirror.

Magnification, m=−υu=−6−30=15 (i.e. < 1)

The positive value of m shows that image erect and its value, which is less than 1, shows that image is smaller than the object. Thus, image is virtual, erect and diminished.

**Question 13:**

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

**Answer 13:**

The positive sign means an image formed by a plane mirror is virtual and erect. Since the magnification is 1 it means that the size of the image is equal to the size of the object.

**Question 14:**

An object 5 cm is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position, nature and size of the image.

**Answer 14:**

Since object size, h = +5 cm,

object distance, u = -20 cm

and radius of curvature, R = +30 cm

A virtual, erect image of height 2.2 cm is formed behind the mirror at a distance of 8.6 cm from the mirror.

**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 focussed 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 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:**

Power of lens (P) = 1/f

P = -2D

f = -1/2 = -0.5 m

A concave lens has a negative focal length. Therefore, 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:**

Power of lens (P) = 1/f

P = 1.5D

f = 1/1.5 = 10/15 = 0.66 m

A convex lens has a positive focal length. Therefore, it is a convex lens or a converging lens.

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