RBSE Solutions for Class10 Science Chapter 9 Light: Reflection and Refraction

In this post, you will get a complete solution for Class 10 Science Chapter 10 Light: Reflection and Refraction contains all the main and important topics which have complete and detailed description.

These solutions will help the students of class 10 to understand concepts.

This post covers complete syllabus and let you secure the best results in RBSE and other board exams

 

 Light: Reflection and Refraction

 

 

Answers of Questions on Page142

Q.1 Define the principal focus of a concave mirror.

Answer   

The principal focus of a concave mirror is a point on its

principal axis to which all the light rays which are parallel

and close to the axis, converge after reflection from the

concave mirror.

 

Q.2 The radius of curvature of a spherical mirror is 20 cm. What is its focal length?

Answer    

Given, radius of curvature (r) = 20 cm 

 

           Focus (F) =`\frac{r}{2}=\frac{20}{2}=10\ cm`

 

Q.3 Name a mirror that can give an erect and enlarged image of an object.

Answer   

Concave mirror can give an erect and enlarged image of

an object.

 

Q.4 Why do we prefer a convex mirror as a rear-view mirror in vehicles?

Answer    

We prefer a convex mirror as a rear view mirror in  vehicles because-

i.        It always forms erect image of an object.

ii.  It covers a wider view and enables us to see a wide area behind vehicles.

 

 

Answers of Questions on Page 145

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

Answer    

Given, radius of curvature (R) = 32 cm

 

            Focal length (f) =`\frac{32}{2}` =     16 cm

Q.2 A concave mirror produces three times magnified real image of an object placed at a distance of 10 cm in front of it. Where is the image located?

Answer       

Given values

u= -10 cm,    

m = -3 (because image is real)

solution

  `\m =\frac {-v}{u}`

 `\-3 =\frac {-v}{10}`

  `\-v=-30 cm`

 

Answers of Questions on Page150

Q.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    

The ray of light will bend towards the normal because the

speed of light decreases when light ray enters from air to

water as  air is optically rarer than water.

 

Q.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  

Given values

 n = 1.5

 c= 3 × 10⁸ ms^-1

 v=?

solution 

 `\n= \frac{c}{v}`

  `\1.5 =\frac{3 × 10^8}{v}`

   v= 3 × 10⁸ ms^-1

Q.3 Find out, from the table 10.3, the medium having highest optical density. Also find the medium with lowest optical density. 

Answer    

Medium having highest optical density is diamond

(refractive index =2.42) and with lowest optical density is

air (refractive index = 1.003)

 

Q.4 You are given kerosene, turpentine and water. In which of these does light travel fastest? Use the information given in table 10.3.

Answer      

In kerosene, turpentine and water, light travels fastest in

water because the speed of light is maximum in the

medium with least refractive index.

 

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

Answer     

The refractive index of diamond is 2.42 means speed of

light is 2.42 times reduced in diamond as compared to

speed of light in vacuum.

 

 

Answers of Questions on Page158

Q.1 Define 1 dioptre of power of a lens.

Answer       

One diopter is the power of a lens whose focal length is 1

metre

 

Q.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 formed is

equal to size of the object ?  Also find the power of the

lens.

Answer  

  Given   values 

  v=50 cm

   h = h^’  

 

 

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

Answer        

 f= - 2m

 

    `p=\frac{1}{f}=\frac{1}{-2}=\ -0.5D` 

 

 

Answers of End Exercise Questions

Q.1 Which one of the following materials cannot be used

to make a lens?

(a)  Water                      

(b)  Glass

(c)  Plastic               

(d)  Clay

Answer       (d)   Clay    

 

 

Q.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       

(d)Between the optical centre of the lens and its principal

focus.

 

Q.3 Where should an object be places 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      (d)  At twice the focal length

 

Q.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 are concave                  

(b)  both are convex

(c)  the mirror is concave and the lens is convex

 (d)  the mirror is convex, but the lens is concave

Answer       (d)  both are concave

 

Q.5 No matters how far you stand from a mirror, your

image appears erect. The mirror is likely to be

 (a)  plane only               

 (b)  concave only

 (c)  convex only

 (d)  either plane or convex

Answer        (d)  either plane or convex

 

Q.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

Ans.      (c)  A convex lens of focal length 5 cm

 

Q.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     

An erect image of an object using a concave mirror of

focal length 15 cm can be obtained when  the object is

placed at a distance less than 15 cm from the pole. Image

formed will be virtual and larger in size than the object.

                          

Q.8 Name the type of mirror used in the following

situations:

 (a)  Head-light of a car

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

 (c)  Solar furnace

   Support your answer with reason.

Answer      

(a)  Concave mirror is used in head-lights of  a car. Bulb

placed at the focus of concave mirror. It reflects the light

rays parallel to get a powerful beam.

(b) Convex mirror is used as side/rear view mirror of a

vehicle because it have a view of much larger area and

moreover it is preferred as they always form an erect

image.

(c) Concave mirrors are used in the solar furnaces they

concentrate sunlight from a larger are to a point and

produce heat.

 

Q.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     

Yes, it will produce a complete image of the object. This

can be verified by an experiment by observing the image

of a distant object like a pole on a screen when lower half

of the lens is covered with a black paper but the intensity

of image will be reduced.

           

                                                       

                                                 

 

Q.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      

Given, height of object (h) = 5 cm.

       Focal length (f) = +10 cm.

       Distance of object (u) = - 25 cm.

 

 

Q.11A 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       

Given values  

focal length of concave lens (f)= -15 cm

Distance of image (v)= -10cm

Distance of object (u) =?

 

 

So, object is placed at a distance of 30 cm on the left side.

 

 

 

Q.12An 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      

Given values

 

Object distance (u) = -10 cm

Focal length of convex mirror (f) = + 15 cm

Image distance (v) for a convex mirror

 

 

 

 

Q.13  The magnification produced by a plane mirror is  +

1. What does this mean?

Answer         

The magnification produced by a plane mirror is + 1

means plane mirror forms virtual image of equal size.

 

Q.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   

Given values

Radius of curvature of convex mirror (r) = + 30 cm.

Object height (h) = 5.0 cm

 Object distance (u) = - 20 cm

 

  

 

 

 

Q.15  An object 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     

  Given values         

Object size (h) = 7.0 cm

Object – distance (u) = -27 cm

Focal length of concave mirror (f) = - 18 cm

 

We know that,

     

 

Q.16 Find the focal length of a concave lens of power – 2.00D

Answer     

Given values 

Power (P)=  -2.00D

focal length (f) =?

 

     

Q.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      

Power