Solution of Chapter 10. Light - Reflection and Refraction (Science Important Questions Book)

Chapter Exercises

Important Questions
Very Short Answer (1 Mark)
Short Answer (2 Marks)
Short Answer (3 Marks)
Long Answer (5 Marks)

Important Questions

1 State the basic laws which determine the reflection of light by mirrors.
2 State Snell's law. Give its uses.
3 Can the laws for plane refracting surfaces be applied to spherical refracting surfaces also? State the laws of refraction of light.
4 What does the positive sign associated with the virtual image and the negative sign associated with the real image denote?
5 State the condition at which total internal reflection occurs.
6 Define the term "star light flux".
7 What impact does atmospheric refraction have on sunrise and sunset?
8 What happens when a ray of light falls normally on the surface of a mirror?
9 What is lateral inversion? Explain with a suitable example.
10 Define principal focus of concave and convex mirror.
11 What is the nature, size and position of the image formed of an object at infinity? Explain how it is used as a doctor's mirror.
12 How will you distinguish between a plane mirror, a concave mirror and a convex mirror without touching them?
13 Explain the path of the light traveling in air incident on a rectangular glass block and emerging out into the air from the opposite face. Draw a ray diagram to show the complete path of this ray of light.
14 An empty test tube is placed slanting in water and viewed from above, what will you observe? What difference will it make when the tube is partially filled with water?
15 Why do stars twinkle on a clear night?
16 A beam of red light is allowed to fall in turn on 
(a) a black cloth 
(b) red cloth 
(c) white cloth 
(d) blue cloth. Describe and explain the appearance in each case.
17 An object is placed between the pole and focus of a concave mirror. What is the nature and size of image?
18 State the condition at which total internal reflection occurs.
19 A stamp collector uses a lens (convex) of focal length 7 cm, to examine a stamp. What is the power of the lens?
20 What is critical angle? Draw a ray diagram showing light incident at the critical angle.
21 Draw the colour circle showing primary and composite colours.
22 What is meant by dispersion of light?
23 How are focal length and radius of curvature connected?
24 An object 5 cm high is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagrams and find the position, size and the nature of the image formed.
25 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.
26 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.
27 What is the angle of incidence when the ray reflected back, when it passes from denser to rarer medium.
28 Draw ray diagrams to show image formation.
Object at C – concave mirror
Object beyond 2f – convex lens
Object at infinity – concave lens. Write the nature of the image in each case.
29 Light, of wavelength of 500 nm in air, enters a glass plate of refractive index 1.5. 
Find   
(i) speed    
(ii) frequency and     
(iii) wavelength of light in glass. 
Assume that the frequency of light remains the same in both media.?
30 Why do we use blue colour solution after washing the clothes?
31 How is critical angle related to the refractive index of the medium?
32 The refractive index of dense flint glass is 1.65, and for alcohol it is 1.36, with respect to air. What is the refractive index of dense flint glass with respect to alcohol?
33 A concave mirror produces 3 times magnified real image of an object placed at 10 cm in front of it. Where is the image located?
34 With the help of labelled ray diagram only show the formation of an image in a compound microscope.
35 Two thin lenses of power +3.5 D and –2.5 D are placed in contact. Find the power and focal length of the lens combination.
36 How can you distinguish between a real image and a virtual image?
37 State Snell's law
38 How image can be located which is formed by a plane mirror?
39 What is the relation between Speed of light in a medium and refractive index. 
40 What happens to light when it enters a rarer medium from a denser medium?
41 Define 1 dioptre.
42 Why are reflecting prisms used in periscopes?
43 Do frequency and wavelength change when light travels from one medium to another?
44 How a virtual image bigger than the object can be produced?
45 Define refractive index in terms of velocity and mass density.
46 Give one application for convex mirror and convex lens.
47 Define power of a lens. For what type of lens is it positive?
48 Distinguish between supplementary and complementary colours. What is composite colour?
49 What is centre of curvature?
50 Define focus of a convex lens. Draw a ray diagram showing the focus.
51 What is a simple microscope?
52 Light of wavelength 500nm in air enters a glass plate of refractive index 1.5. find its wavelength in glass.
53 What is the consequence of making angle of incidence of light, at an interface, greater than the critical angle?
54 With the help of a labeled ray diagram show the formation of image of an object by a compound microscope.
55 What is meant by refraction?
56 Express the refractive index of the medium.
57 If a ray of light passes from medium I to medium II without any change of direction, what can be said about the refractive indices of these media.
58 Mention two properties of a wave: one property which varies and the other which remains constant when the wave passes from one medium to another.
59 Name one factor on which the direction of bending of a ray of light depends.
60 A ray of light while traveling from a denser to a rarer medium, is incident at critical angle on the interface. What is the angle of refraction in the rarer medium?
61 A glass slab is placed over a page in which letters are printed in different colours. Will the image of all the letters lie in the same plane?
62 Does the apparent depth of a tank of water change, normally.
63 For which colour of white light is the refractive index of a transparent medium is maximum and minimum.
64 What is total reflecting prism?
65 State three actions that a total reflecting prism can produce.
66 Mention the types of lenses.
67 What is a convex lens?
68 Mention the types convex lens.
69 What is radius of curvature?
70 What is optical centre?
71 What is pupil?
72 What is aqueous houmour?
73 What is vitreous humour?
74 What is far point?
75 What is cornea?
76 What is retina?
77 What is blind spot?
78 What is eye lens?
79 What is iris?
80 Define magnification power of a simple microscope.
81 How to increase the magnification and sharpness of the image in optical instruments?
82 Optical density. Explain
83 Explain diffraction of light.
84 Give some of the uses of convex mirrors.
85 How far will a light beam travel? If there was an extremely strong light beam that could be seen for miles, where would the light stop? If a flashlight has low batteries, how far will its light beam travel?
86 Give the applications of plane mirror.
87 Give the expression for refractive index in terms of speed of light.

Very Short Answer (1 Mark)

1

Define the principal focus of a concave mirror.
2

The magnification produced by a plane minor is +L What does this mean ?
3

Find the power of a concave lens of focal length 2 m.
4

What does the negative sign in the value of magnification produced by a mirror indicates about an image?
5

Given below are refractive indices of a few material media.

Rock salt - 1.54


Ice - 1.31


Dense flint glass - 1.65


Crown glass - 1.52


Arrange the media in the ascending order of optical density.
6

Define 1 Dioptre of power of a lens.
7

Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? 1 speed of light in vacuum is 3 x 108 m s-1
8

You are given kerosene, turpentine and water. In which of these the light travels fastest?
9

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?
10

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

Short Answer (2 Marks)

11

Why do we prefer a convex mirror as a rear-view mirror in vehicles?
12

A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal away from the normal? Why?
13

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 in size to the object? Also find the power of the lens.
14

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.
15

What are laws of refraction of light?
16

State the relation between object distance, image distance and focal length of a spherical mirror.
17

An object is placed at a distance of 15 cm from a convex lens of focal length 20 cm. List four characteristics (nature, position, etc.) of the image formed by the lens.
18

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?
19

A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object from the lens ? Draw the ray diagram.
20

An erect image three times the size of the object is formed with a concave mirror of radius of curvature 36 cm. What is the position of the image?
21

For driving a car what type of mirror would you prefer to see the traffic at your back and why? Explain why it is preferred over a plane mirror?

Short Answer (3 Marks)

22

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 nature of the image.
23

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 nature of the image formed.
24

Light passes through a rectangular glass slab and through a triangular glass prism. Using proper ray diagram, explain in what way does the direction of the two emergent beams differs with respect to the incident beam of light.
25

The image of an object formed by a lens is real, inverted and of the same size as the object. If the image is at a distance of 40 cm from the lens, what is the nature and power of the lens? Draw ray diagram to justify your answer.
26

The image of an object formed by a lens is real, inverted and of the same size as the object. If the image is at a distance of 40 cm from the lens, what is the nature and power of the lens? Draw ray diagram to justify your answer.
27

Draw a ray diagram to show why a stick half immersed in water appears to be bent at the surface.
28

Find the position, nature and size of the image of an object 3 cm high place at a distance of 9 cm from a concave mirror of focal length 18 cm.

Long Answer (5 Marks)

29

You have two lenses A and B of focal lengths + 10 cm and – 10 cm respectively. State the nature and power of each lens. Which of the two lenses will form a virtual and magnified image of an object placed 8 cm from the lens? Draw a ray diagram to justify your answer.
30

Draw the ray diagram to show the formation of image by a concave mirror of focal length 15 cm for the following positions of object. (Diagrams may not be drawn to the scale.)

Indicate is placed at 30 cm from mirror.


a. object is placed at 30 cm from mirror.


b. object is placed at 10 cm form the mirror.