# NCERT Solutions for Class 10 Science Unit 10

## Light - Reflection and Refraction Class 10

### Unit 10 Light - Reflection and Refraction Exercise Solutions

Reflection :- The bouncing back of rays of light from a polished and shiny surface is called reflection or reflection of light. It is similar to bouncing back of a football after colliding with a wall or any hard surface.

Refraction :- The change of direction of light because of change of medium is known as Refraction or Refraction of Light. The ray of light changes its direction or phenomenon of refraction takes place because of difference in speed in different media. The light travels at faster speed in rare medium and at slower speed in denser medium. The nature of media is taken as relative. For example air is a rarer medium than water or glass. When ray of light enters from a rarer medium into a denser medium, it bends towards normal at the point of incidence. On the contrary, when ray of light enters into a rarer medium from a denser medium it bends away from the normal. Ray emerging after the denser medium goes in the same direction and parallel to the incident ray.

### Question : Solutions of Questions on Page Number : 168

Q1 :

Define the principal focus of a concave mirror.

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.

Q2 :

The radius of curvature of a spherical mirror is 20 cm. What is its focal length?

Radius of curvature, R= 20 cm

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

R= 2f

Hence, the focal length of the given spherical mirror is 10 cm.

Q3 :

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

Q4 :

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

Q5 :

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

Q6 :

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?

Q7 :

Aray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?

Q8 :

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 m s-1.

Q9 :

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

 Material medium Refractive index Material medium Refractive index Air 1.0003 Canada Balsam 1.53 Ice 1.31 - - Water 1.33 Rock salt 1.54 Alcohol 1.36 - - Kerosene 1.44 Carbon disulphide 1.63 Fused quartz 1.46 Dense flint glass 1.65 Turpentine oil 1.47 Ruby

Q10 :

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

 Material medium Refractive index Material medium Refractive index Air 1.0003 Canada Balsam 1.53 Ice 1.31 - - Water 1.33 Rock salt 1.54 Alcohol 1.36 - - Kerosene 1.44 Carbon disulphide 1.63 Fused quartz 1.46 Dense flint glass 1.65 Turpentine oil 1.47 Ruby 1.71 Benzene 1.50 Sapphire 1.77

Q11 :

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

Q12 :

Define 1 dioptre of power of a lens.

Q13 :

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.

Q14 :

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

### Exercise : Solutions of Questions on Page Number : 185

Q1 :

Which one of the following materials cannot be used to make a lens?

(a) Water

(b) Glass

(c) Plastic

(d) Clay

Q2 :

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.

Q3 :

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.

Q4 :

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

Q5 :

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

Q6 :

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

Q7 :

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.

Q8 :

Name the type of mirror used in the following situations.

(b) Side/rear-view mirror of a vehicle

(c) Solar furnace

Q9 :

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.

Q10 :

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.

Q11 :

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.

Q12 :

An object is placed at a distance of 10 cm from aconvex mirror of focal length 15 cm. Find the position and nature of the image.

Q13 :

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

Q14 :

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.

Q15 :

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.

Q16 :

Find the focal length of a lens of power -2.0 D. What type of lens is this?

Q17 :

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?