In this post, you will get a complete solution for Class 10 Science Chapter 9 Light: Reflection and Refraction contains all the main and important topics which have complete and detailed description.
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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
(F) = r/2 = 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) =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 = -v / u
-3 = -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 = 3 ×108 /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=1/f}= 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
RBSE Class 10 Science Questions Answers
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