CBSE Class X

Question 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:

Range of object distance = 0 cm to15 cm

A concave mirror gives an erect image when an object is placed between its pole

(P) and the principal focus (F).

Hence, to obtain an erect image of an object from a concave mirror of focal length 15

cm, the object must be placed anywhere between the pole and the focus. The image

formed will be virtual, erect, and magnified in nature, as shown in the given figure.

Question 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

Answer:

(c) A convex lens gives a magnified image of an object when it is placed between

the radius of curvature and focal length. Also, magnification is more for convex

lenses having shorter focal length. Therefore, for reading small letters, a convex

lens of focal length 5 cm should be used.

Question 5:

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

Answer:

(d) A convex mirror always gives a virtual and erect image of smaller size of the

object placed in front of it. Similarly, a plane mirror will always give a virtual and

erect image of same size as that of the object placed in front of it. Therefore, the

given mirror could be either plane or convex.

Question 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 concave

(b) both convex

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

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

Answer:

(a) By convention, the focal length of a concave mirror and a concave lens are taken

as negative. Hence, both the spherical mirror and the thin spherical lens are concave

in nature.

Question 3:

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.

Answer:

(b) When an object is placed at the centre of curvature in front of a convex lens, its

image is formed at the centre of curvature on the other side of the lens. The image

formed is real, inverted, and of the same size as the object.

Question 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) When an object is placed between the pole and principal focus of a concave

mirror,the image formed is virtual, erect, and larger than the object.

Question 1:

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

(a) Water

(b) Glass

(c) Plastic

(d) Clay

Answer:

(d) A lens allows light to pass through it. Since clay does not show such property,

it cannot be used to make a lens.

Question 3:

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

Answer:

Focal length of concave lens, f = 2 m

Here, negative sign arises due to the divergent nature of concave lens.

Hence, the power of the given concave lens is −0.5 D.

Question 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 is equal

to the size of the object? Also, find the power of the lens.

Answer:

When an object is placed at the centre of curvature, 2F₁, of a convex lens, its image

is formed at the centre of curvature, 2F₂, on the other side of the lens. The image

formed is inverted and of the same size as the object, as shown in the given figure.

It is given that the image of the needle is formed at a distance of 50 cm from the

convex lens. Hence, the needle is placed in front of the lens at a distance of 50 cm.

Object distance, u = −50 cm

Image distance, v = 50 cm

Focal length = f

According to the lens formula,

Hence, the power of the given lens is +4 D.

Question 1:

Define 1 dioptre of power of a lens.

Answer:

Power of lens is defined as the reciprocal of its focal length. If P is the power of a

lens of focal length F in metres, then

The S.I. unit of power of a lens is Dioptre. It is denoted by D.

1 dioptre is defined as the power of a lens of focal length 1 metre.

∴1 D = 1 m⁻¹

Question 5:

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

Answer:

Refractive index of a medium nm is related to the speed of light in that medium v

by the relation:

Where, c is the speed of light in vacuum/air

The refractive index of diamond is 2.42. This suggests that the speed of light in

diamond will reduce by a factor 2.42 compared to its speed in air.

Question 4:

You are given kerosene, turpentine and water. In which of these does the light

travel fastest? Use the information given in Table.

Answer:

Speed of light in a medium is given by the relation for refractive index (n_m). The

relation is given as

It can be inferred from the relation that light will travel the slowest in the material

which has the highest refractive index and travel the fastest in the material which

has the lowest refractive index.

It can be observed from table 10.3 that the refractive indices of kerosene, turpentine,

and water are 1.44, 1.47, and 1.33 respectively. Therefore, light travels the fastest in

water.

Question 3:

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

medium with lowest optical density.

Answer:

Highest optical density = Diamond

Lowest optical density = Air

Optical density of a medium is directly related with the refractive index of that

medium.

A medium which has the highest refractive index will have the highest optical

density and vice-versa.

It can be observed from table 10.3 that diamond and air respectively have the

highest and lowest refractive index. Therefore, diamond has the highest optical

density and air has the lowest optical density.

Question 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:

Refractive index of a medium n_m is given by,

Speed of light in vacuum, c = 3 × 10⁸ m s⁻¹ 

Refractive index of glass, n_g = 1.50

Speed of light in the glass, 

Question 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 light ray bends towards the normal.

When a ray of light travels from an optically rarer medium to an optically denser

medium, it gets bent towards the normal. Since water is optically denser than air,

a ray of light travelling from air into the water will bend towards the normal.

Question 2:

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?

Answer:

Magnification produced by a spherical mirror is given by the relation,

Let the height of the object, h_o = h

Then, height of the image, hI = −3h (Image formed is real)

Object distance, u = −10 cm

v = 3 × (−10) = −30 cm

Here, the negative sign indicates that an inverted image is formed at a distance of

30 cm in front of the given concave mirror.

Question 1:

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

Answer:

Radius of curvature, R = 32 cm

Radius of curvature = 2 × Focal length (f)

R = 2f

Hence, the focal length of the given convex mirror is 16 cm.

Question 4:

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

Answer:

Convex mirrors give a virtual, erect, and diminished image of the objects placed in

front of them. They are preferred as a rear-view mirror in vehicles because they give

a wider field of view, which allows the driver to see most of the traffic behind him.

Question 3:

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

Answer:

When an object is placed between the pole and the principal focus of a concave

mirror,the image formed is virtual, erect, and enlarged.

Question 2:

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

Answer:

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.