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Question-1

Define the principle focus of a concave mirror?

Solution:
The principle focus of a concave mirror is a point on its principle axis to which all the light rays which are parallel and close to the axis, converge after reflection from the concave mirror.

Question-2

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

Solution:

Here focal length f = ? (To be calculated)

And radius of curvature, R=20 cm

So, = 10 cm

Thus the focal length of a spherical mirror is 10 cm.

Question-3

Name a mirror that can give an erect and enlarged image of an object?

Solution:
Concave mirror can give an erect and enlarged image of an object.

Question-4

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

Solution:
Convex mirrors are preferred because they always give an erect, though diminished, image. Also they have a wider field of view as they are curved outward. Thus, convex mirror enable the driver to view much larger area.

Question-5

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

Solution:
We know that,

And the focal length, f = ?

And radius of curvature, R = 32 cm

So, = 16 cm

Thus the focal length of this convex mirror is 16 cm.

Question-6

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?

Solution:

M = 3

We know that,

U = -(10/3)

   = - 3.3 × 101 m.

Question-7

A ray of light traveling in air enters obliquely in to water. Does the light ray bend towards the normal or away from the normal? Why?

Solution:
The light ray bends towards the normal because water is an optically denser medium than air. Whenever a ray of light goes from a rarer medium to a denser medium it bends towards the normal.

Question-8

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 3x108 ms-1.

Solution:
We know that,



= 2 × 108 m/s
Thus the speed of light in glass is 2 × 108 m/s.

Question-9

Find out from this table the medium having highest optical density. Also find the medium with lowest optical density.

Solution:

Diamond has highest optical density
And Air has lowest optical density.

Question-10

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

Solution:

The light can travel fast through water.

Question-11

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

Solution:
Diamond having higher refractive index, it is optically denser than other material mediums like air, water etc.

Question-12

Define 1 dioptre of power of a lens.

Solution:
The S.I unit of the power of a lens is dioptre, which is denoted by the letter D. One dioptre is the power of a lens whose focal length is 1 metre.

Question-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 to the size of the object? Also find the power of the lens.

Solution:
The needle should be placed at 2F in front of the convex lens.

f =+50 cm
= +2 D
therefore the power of the lens is +2 D.

Question-14

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

Solution:
f=2 m
=-0.5 D
Power of the concave lens is -0.5 D.

Question-15

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

(b) glass

(c) plastic

(d) clay
.

Solution:
(d) clay.

Question-16

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.


Solution:
(d) Between the pole of the mirror and its principal focus.

Question-17

Where an object should 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.


Solution:
(b) at twice the focal length.

Question-18

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.

 


Solution:
(a) both concave.

Question-19

No matter how far you stand from a mirror, your image appear erect. The mirror is likely to be
(a) Plane

(b) Concave

(C) Convex

(d) Either plane or convex.


Solution:
(a) plane.

Question-20

Which of these 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.


Solution:
(a) a convex lens of focal length 50 cm.

Question-21

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.

Solution:
When object is between principal focus and pole of a concave mirror, an erect, enlarged and virtual image is formaed. So, we need to keep the object at a distance which is less than 15 cm (the given focal length).

Question-22

Name the type of mirror used in the following situation
(a) Headlights of a car.

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

(c) Solar furnace.

Support your answer with reason.


Solution:
(a) Concave mirrors is used in headlights of a car to get powerful parallel beams of light.

(b) Convex mirrors are commonly used as rear-view mirrors in vehicles because they always give an erect, though diminished, image. Also they have a wider field of view as they are curved outward.

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

Question-23

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.

Solution:
No, the lens will not produce the complete image of the object. If an object is placed in front 0f the convex lens and one-half of the lens is covered by a black paper, the image of the object cannot be obtained because the rays cannot pass through the convex lens properly. To verify experimentally, If we place a needle in front of the convex lens at a distance of 10 cm. one-half of the lens is covered by a black paper, the complete image of the needle cannot be seen, because the rays cannot pass through the lens.

Question-24

An Object 5 cm in length is held in 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.

Solution:
Object distance u = -25 cm
Image distance v = ?
Focal length f = 10 cm
Using the lens formula


   
 
 

V = 16.67 cm


Using the Lens Formula
Positive sign shows that the image formed is to the right of the lens and is at 16.67 cm from the lens, i.e., the image is formed between F and 2f. Since only the real and inverted image is formed on the right hand side of a convex lens, therefore, the image formed is real and inverted.
Magnification,
Again

where h1 and h2 are the sizes of objects and images

i.e., (h1 is always positive)
h2 = - 3.35 cm
Size of image is 3.35 cms, hence smaller than the object and is always inverted.

Question-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.

Solution:
Focal length of the lens f = -15 cm (since concave lens are negative)
Since the image is formed on the same side, v = -10 cm.
u = ?

Using Lens Formula

                   
 

Therefore u = -30 cm
 

Question-26

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

Solution:
Object distance, u = -10 cm
Image distance, v = ?
Focal length, f = -15cm

Using the Mirror Formula

    

     

        =
-2 +3 /30 = 1/30

V= 30 cm

The image is at a distance of 6 cm behind the mirror. It is virtual, erect and magnified.

Question-27

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

Solution:
Magnification produced by a plane mirror is +1 means that image distance is equal to the object distance.

Question-28

An object of 5.0 cm in length is placed at a distance of 20 cm in front of the convex mirror of radius of curvature 30 cm. find the position of the image, its nature and size.

Solution:
Object distance, u = - 20 cm

Image distance, v = ?

Focal length,   = = 15 cm.

Using the Mirror Formula




 
v = 8.6 cm
Magnification
= 0.43
Height of the object, h1 = +5 cm Height of the image, h2 = ?


h2 = 0.43 × 5

Height of the magnified image, h2 = 2.15 cm.

Question-29

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.

Solution:
Object distance u = -27 cm
Image distance v = ?
Focal length of the mirror f = - 18 cm

Using Mirror Formula




v = -54 cm
Magnification
where height of the object h1 = 7 cm

         

= -14 cm
Negative sign indicates that the image is on the same side as that of the object. It is real, inverted and 14 cm in size.

Question-30

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

Solution:
Power of the concave lens = -2.0 D
= -0.5 m.

As the f is negative it is a concave lens

Question-31

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?

Solution:
Power = +1.5 D

   
      = 66.6 cm
The focal length of the lens is 66.6 cm. The lens is converging.
 




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