Chapter 5

Refraction Through a Lens

Class 10 – Concise Physics Selina 5(a) and 5(c)

Exercise 5(A)

Question 1

What is a lens?

Answer

A lens is a transparent refracting medium bounded by either two spherical surfaces or one surface spherical and another surface plane.

There are two types of lenses —

  1. Convex or converging lens
  2. Concave or diverging lens

Question 2

Name the two kinds of lens? Draw diagrams to illustrate them.

Answer

Two types of lenses are —

  1. Convex or converging lens
  2. Concave or diverging lens
Name the two kinds of lens? Draw diagrams to illustrate them. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 3

State difference between a convex and a concave lens in their —

(a) appearance, and

(b) action on the incident light.

Answer

(a) The difference between a convex and a concave lens in terms of appearance is as follows —

Convex lens Concave lens
A convex lens is thick in the middle and thin at its periphery. A concave lens is thin in the middle and thick at its periphery.
It bulges out in the middle. It is bent inwards in the middle.

(b) The difference between a convex and a concave lens in terms of action on the incident light is as follows —

Convex lens Concave lens
A convex lens converges the incident rays towards the principal axis. A concave lens diverges the incident rays away from the principal axis.

Question 4

Which lens is converging —

(i) an equiconcave lens or an equiconvex lens?

(ii) a concavo-convex lens or a convexo-concave lens?

Answer

(i) An equiconvex lens or bi convex has both its surfaces convex, and is a converging lens.

(ii) A concavo-convex lens is thicker in the middle and thinner at the edges and has a converging action on a light beam. Hence, a concavo-convex lens is a converging lens.

Question 5

Out of the two lenses, one concave and the other convex, state which one will show the divergent action on a light beam. Draw diagrams to illustrate your answer.

Answer

A concave lens is thick at its periphery and thin in the middle. In other words, a lens which is bent inwards in the middle, is the concave lens. Such a lens diverges the light rays incident on it, so it is called a diverging lens.

Out of the two lenses, one concave and the other convex, state which one will show the divergent action on a light beam. Draw diagrams to illustrate your answer. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 6

Show by a diagram the refraction of two light rays incident parallel to the principal axis on a convex lens by treating it as a combination of a glass slab and two triangular glass prisms.

Answer

Below diagram shows the refraction of two light rays incident parallel to the principal axis on a convex lens. The convex lens is shown as a combination of a glass slab and two triangular glass prisms:

Show by a diagram the refraction of two light rays incident parallel to the principal axis on a convex lens by treating it as a combination of a glass slab and two triangular glass prisms. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 7

Show by a diagram the refraction of two light rays incident parallel to the principal axis on a concave lens by treating it as a combination of a glass slab and two triangular glass prisms.

Answer

Below diagram shows the refraction of two light rays incident parallel to the principal axis on a concave lens. The concave lens is shown as a combination of a glass slab and two triangular glass prisms:

Show by a diagram the refraction of two light rays incident parallel to the principal axis on a convex lens by treating it as a combination of a glass slab and two triangular glass prisms. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 8

How does the action of the convex lens differ from that of a concave lens on a parallel beam of light incident on them? Draw diagrams to illustrate your answer.

Answer

Convex Lens Concave Lens
In the case, when a parallel beam of light ray is incident on a convex lens, the prism in the upper part bends the incident ray downwards. In the case, when a parallel beam of light ray is incident on a concave lens, the upper part of the lens bends the incident ray upwards.
The ray incident on the lower part of the prism is bent in upward direction. The ray incident on the lower part of the prism is bent in downward direction
The central part which is a parallel-sided glass slab passes the ray undeviated. The central part which is a parallel-sided glass slab passes the ray undeviated.

Convergent action of Convex lens is shown in the below diagram:

How does the action of the convex lens differ from that of a concave lens on a parallel beam of light incident on them? Draw diagrams to illustrate your answer. Refraction through a lens, Concise Physics Class 10 Solutions.

Divergent action of Concave lens is shown in the below diagram:

How does the action of the convex lens differ from that of a concave lens on a parallel beam of light incident on them? Draw diagrams to illustrate your answer. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 9

Define the term principal axis of a lens.

Answer

The line joining the centres of curvature of the two surfaces of the lens is called the principal axis.

Question 10

Explain the optical centre of a lens with the help of proper diagram(s).

Answer

Optical centre is a point on the principal axis of the lens such that a ray of light passing through this point emerges parallel to its direction of incidence. The optical centre is thus the centre of the lens.

Explain the optical centre of a lens with the help of proper diagram(s). Refraction through a lens, Concise Physics Class 10 Solutions.

The point O is the optical centre of the convex and concave lenses .

Question 11

A ray of light incident at a point on the principal axis of a convex lens passes undeviated through the lens.

(a) What special name is given to this point on the principal axis?

(b) Draw a labelled diagram to support the answer in part (a).

Answer

(a) This point on the principal axis is called Optical Centre.

(b) Below labelled diagram shows the Optical Centre for Convex and Concave lens:

A ray of light incident at a point on the principal axis of a convex lens passes undeviated through the lens. What special name is given to this point on the principal axis? Draw a labelled diagram to support the answer in part a. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 12

State the condition when a lens is called equiconvex or equi-concave.

Answer

A lens is called an equiconvex or equi-concave lens, when the radii of curvature of the two surfaces of lens are equal.

Question 13

Define the term principal foci of a convex lens and illustrate your answer with the aid of proper diagrams.

Answer

A light ray can enter a lens from either side, therefore, a lens has two principal foci, one on either side of the lens. These are known as first focal point( or first focus) F1 and the second focal point ( or second focus ) F2

For a convex lens, the first focal point is a point F1 on the principal axis of the lens such that the rays of light coming from it, after refraction through the lens, become parallel to the principal axis of the lens.

Define the term principal foci of a convex lens and illustrate your answer with the aid of proper diagrams. Refraction through a lens, Concise Physics Class 10 Solutions.

For a convex lens, the second focal point is a point F2 on the principal axis of the lens such that the rays of light incident parallel to the principal axis, after refraction from the lens, pass through it.

Define the term principal foci of a convex lens and illustrate your answer with the aid of proper diagrams. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 14

Define the term principal foci of a concave lens and show them with the help of proper diagrams.

Answer

A light ray can enter a lens from either side, therefore, a lens has two principal foci, one on either side of the lens. These are known as first focal point( or first focus) F1 and the second focal point ( or second focus ) F2

For a concave lens, the first focal point is a point F1 on the principal axis of a lens such that the incident rays of light appearing to meet at it, after refraction from the lens become parallel to the principal axis of the lens.

Define the term principal foci of a concave lens and show them with the help of proper diagrams. Refraction through a lens, Concise Physics Class 10 Solutions.

For a concave lens, the second focal point is a point F2 on the principal axis of the lens such that the rays of light incident parallel to the principal axis, after refraction from the lens, appear to be diverging from this point.

Define the term principal foci of a concave lens and show them with the help of proper diagrams. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 15

Draw a diagram to represent the second focus of a concave lens.

Answer

Below diagram represents the second focus of a concave lens:

Draw a diagram to represent the second focus of a concave lens. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 16

Draw a diagram to represent the second focus of a convex lens.

Answer

Below diagram represents the second focus of a convex lens:

Draw a diagram to represent the second focus of a convex lens. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 17

A ray of light, after refraction through a concave lens emerges parallel to the principal axis.

(a) Draw a ray diagram to show the incident ray and its corresponding emergent ray.

(b) The incident ray when produced meets the principal axis at a point F. Name the point F.

Answer

(a) Below diagram shows the incident ray and its corresponding emergent ray:

Define the term principal foci of a concave lens and show them with the help of proper diagrams. Refraction through a lens, Concise Physics Class 10 Solutions.

(b) The incident ray when produced meets the principal axis at a point F, which is called as the first focus .

Question 18

A ray of light, after refraction through a convex lens emerges parallel to the principal axis.

(a) Draw a ray diagram to show it.

(b) The incident ray passes through a point F on the principal axis. Name the point F.

Answer

(a) Below ray diagram shows the incident ray and its corresponding emergent ray:

A ray of light, after refraction through a convex lens emerges parallel to the principal axis. Draw a ray diagram to show it. The incident ray passes through a point F on the principal axis. Name the point F. Refraction through a lens, Concise Physics Class 10 Solutions.

(b) The point through which the incident ray passes is known as the first focus.

Question 19

A beam of light incident on a convex lens parallel to its principal axis converges at a point F on the principal axis. Name the point F. Draw a ray diagram to show it.

Answer

The point at which a beam of light incident on a convex lens parallel to its principal axis converges is called the second focus. Below ray diagram shows the second focus of a convex lens:

Draw a diagram to represent the second focus of a convex lens. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 20

A beam of light incident on a thin concave lens parallel to its principal axis diverges and appears to come from a point F on the principal axis. Name the point F. Draw a ray diagram to show it.

Answer

The point at which a beam of light incident on a concave lens parallel to its principal axis converges is called the second focus. Below ray diagram shows the second focus of a concave lens:

A beam of light incident on a thin concave lens parallel to its principal axis diverges and appears to come from a point F on the principal axis. Name the point F. Draw a ray diagram to show it. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 21

Define the term focal length of a lens.

Answer

The distance of focus from the optical centre of the lens, is called its focal length.

A lens has two focal length.

  1. First focal length — the distance from the optical centre O of the lens to its first focal point F1 is called the first focal length.
  2. Second focal length — the distance from the optical centre O of the lens to its second focal point F2 is called the second focal length.

Question 22

What do you mean by focal plane of a lens?

Answer

A plane normal to the principal axis, passing through the focus, is called the focal plane of a lens.

A lens has two focal plane.

(a) A plane passing through the first focal point and normal to the principal axis of the lens is called the first focal plane.

(a) A plane passing through the second focal point and normal to the principal axis of the lens is called the second focal plane.

Question 23

State the condition for each of the following :

(a) a lens has both its focal lengths equal.

(b) a ray passes undeviated through the lens.

Answer

(a) When the medium is the same on either side of the lens then it has both its focal lengths equal.

(b) A ray passes undeviated through the lens, when it is incident at the optical centre of the lens.

Question 24

A parallel oblique beam of light falls on a

(i) convex lens,

(ii) concave lens.

Draw a diagram in each case to show the refraction of light through the lens.

Answer

(i) Below diagram shows the refraction of a parallel oblique beam of light by a convex lens:

A parallel oblique beam of light falls on a convex lens. Draw a diagram to show the refraction of light through the lens. Refraction through a lens, Concise Physics Class 10 Solutions.

(ii) Below diagram shows the refraction of a parallel oblique beam of light by a concave lens:

A parallel oblique beam of light falls on a concave lens. Draw a diagram to show the refraction of light through the lens. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 25

The diagram below shows a lens as a combination of a glass slab and two prisms.

The diagram shows a lens as a combination of a glass slab and two prisms. Name the lens formed by the combination. Complete the ray diagram and show the path of the incident ray AB after passing through the lens. Refraction through a lens, Concise Physics Class 10 Solutions.

(i) Name the lens formed by the combination.

(ii) What is the line XX’ called?

(iii) Complete the ray diagram and show the path of the incident ray AB after passing through the lens.

(iv) The final emergent ray will either meet XX’ at a point or appear to come from a point on XX’. Label the point as F. What is this point called?

Answer

(i) The lens formed by the combination is a convex lens.

(ii) The line XX’ is called the principal axis.

(iii) Below diagram shows the path of the incident ray AB after passing through the convex lens:

The diagram shows a lens as a combination of a glass slab and two prisms. Name the lens formed by the combination. Complete the ray diagram and show the path of the incident ray AB after passing through the lens. Refraction through a lens, Concise Physics Class 10 Solutions.

(iv) The point F is called a focal point.

Question 26

The diagram below shows a lens as a combination of a glass slab and two prisms.

The diagram shows a lens as a combination of a glass slab and two prisms. Name the lens formed by the combination. What is the line XX' called? Complete the path of the incident ray AB after passing through the lens. The final emergent ray either meets XX' at a point or appears to come from a point on XX’. Label it as F. What is this point called? Refraction through a lens, Concise Physics Class 10 Solutions.

(i) Name the lens formed by the combination.

(ii) What is the line XX’ called?

(iii) Complete the path of the incident ray AB after passing through the lens.

(iv) The final emergent ray either meets XX’ at a point or appears to come from a point on XX’. Label it as F. What is this point called?

Answer

(i) The combination forms a concave lens.

(ii) The line XX’ is called the principal axis.

(iii) Below diagram shows the path of the incident ray AB after passing through the concave lens:

The diagram shows a lens as a combination of a glass slab and two prisms. Name the lens formed by the combination. What is the line XX' called? Complete the path of the incident ray AB after passing through the lens. The final emergent ray either meets XX' at a point or appears to come from a point on XX’. Label it as F. What is this point called? Refraction through a lens, Concise Physics Class 10 Solutions.

(iv) The point F is called a focal point or focus.

Question 27

In figure (a) and (b), F1 and F2 are the positions of the two foci of thin lenses. Draw the path taken by the light ray AB after it emerges from each lens.

(a) Figure a

In figure (a) and (b), F1 and F2 are the positions of the two foci of thin lenses. Draw the path taken by the light ray AB after it emerges from each lens. Refraction through a lens, Concise Physics Class 10 Solutions.

(b) Figure b

In figure (a) and (b), F1 and F2 are the positions of the two foci of thin lenses. Draw the path taken by the light ray AB after it emerges from each lens. Refraction through a lens, Concise Physics Class 10 Solutions.

Answer

(a) Below ray diagram shows the path taken by the light ray AB after it emerges from the thin concave lens:

In figure (a) and (b), F1 and F2 are the positions of the two foci of thin lenses. Draw the path taken by the light ray AB after it emerges from each lens. Refraction through a lens, Concise Physics Class 10 Solutions.

(b) Below ray diagram shows the path taken by the light ray AB after it emerges from the thin convex lens:

In figure (a) and (b), F1 and F2 are the positions of the two foci of thin lenses. Draw the path taken by the light ray AB after it emerges from each lens. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 28

In figure (a) and (b), F1 and F2 are the two foci of thin lenses and AB is the incident ray. Complete the diagram to show the path of the ray AB after refraction through the lens.

(a) Figure a

In figure (a) and (b), F1 and F2 are the positions of the two foci of thin lenses and AB is the incident ray. Complete the diagram to show the path of the ray AB after refraction through the lens. Refraction through a lens, Concise Physics Class 10 Solutions.

(b) Figure b

In figure (a) and (b), F1 and F2 are the positions of the two foci of thin lenses and AB is the incident ray. Complete the diagram to show the path of the ray AB after refraction through the lens. Refraction through a lens, Concise Physics Class 10 Solutions.

Answer

(a) Below ray diagram shows the path taken by the light ray AB after refraction through the thin convex lens:

In figure (a) and (b), F1 and F2 are the positions of the two foci of thin lenses and AB is the incident ray. Complete the diagram to show the path of the ray AB after refraction through the lens. Refraction through a lens, Concise Physics Class 10 Solutions.

(b) Below ray diagram shows the path taken by the light ray AB after refraction through the thin concave lens:

In figure (a) and (b), F1 and F2 are the positions of the two foci of thin lenses and AB is the incident ray. Complete the diagram to show the path of the ray AB after refraction through the lens. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 29

Complete the following sentences —

(a) If half part of a convex lens is covered, the focal length __________ change, but the intensity of image _________.

(b) A convex lens is placed in water. Its focal length will ________.

(c) The focal length of a thin convex lens is _________ than that of a thick convex lens.

Answer

(a) If half part of a convex lens is covered, the focal length does not change, but the intensity of the image decreases.

(b) A convex lens is placed in water. Its focal length will increase.

(c) The focal length of a thin convex lens is more than that of a thick convex lens.

Multiple Choice Type

Question 1

A ray of light after refraction through a lens emerges parallel to the principal axis of the lens. The incident ray either passes through :

  1. its optical centre
  2. its first focus ✓
  3. its second focus
  4. the centre of curvature of the first surface

Answer

When the incident ray passes through the first focus then it emerges parallel to the principal axis of the lens after refraction. We can understand this better with the help of diagrams given below —

Convex Lens

When the incident ray passes through the first focus then it emerges parallel to the principal axis of the lens after refraction. Refraction through a lens, Concise Physics Class 10 Solutions.

Concave Lens

When the incident ray passes through the first focus then it emerges parallel to the principal axis of the lens after refraction. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 2

A ray of light incident on a lens parallel to its principal axis, after refraction passes through or appears to come from:

  1. its first focus
  2. its optical entre
  3. its second focus ✓
  4. the centre of curvature of its second surface.

Answer

A ray of light incident on a lens parallel to its principal axis, after refraction passes through or appears to come from its second focus. We can understand this better with the help of the diagrams given below —

Convex Lens

A ray of light incident on a lens parallel to its principal axis, after refraction passes through or appears to come from its second focus. Refraction through a lens, Concise Physics Class 10 Solutions.

Concave Lens

A ray of light incident on a lens parallel to its principal axis, after refraction passes through or appears to come from its second focus. Refraction through a lens, Concise Physics Class 10 Solutions.

Exercise 5(B)

Question 1

What are the three principal rays that are drawn to construct the ray diagram for the image formed by a lens? Draw diagrams to support your answer.

Answer

(i) A ray of light incident at the optical centre O of the lens passes undeviated through the lens.

Convex Lens

What are the three principal rays that are drawn to construct the ray diagram for the image formed by a lens? Draw diagrams to support your answer. Refraction through a lens, Concise Physics Class 10 Solutions.

Concave Lens

What are the three principal rays that are drawn to construct the ray diagram for the image formed by a lens? Draw diagrams to support your answer. Refraction through a lens, Concise Physics Class 10 Solutions.

(ii) A ray of light incident parallel to the principal axis of the lens, after refraction passes through the second focus F2 (in a convex lens) or appears to come from the second focus F2 (in a concave lens), respectively.

Convex Lens

What are the three principal rays that are drawn to construct the ray diagram for the image formed by a lens? Draw diagrams to support your answer. Refraction through a lens, Concise Physics Class 10 Solutions.

Concave Lens

What are the three principal rays that are drawn to construct the ray diagram for the image formed by a lens? Draw diagrams to support your answer. Refraction through a lens, Concise Physics Class 10 Solutions.

(iii) A ray of light passing through the first focus F1 (in a convex lens) or directed towards the first focus F1 (in a concave lens), emerges parallel to the principal axis after refraction, respectively.

Convex Lens

What are the three principal rays that are drawn to construct the ray diagram for the image formed by a lens? Draw diagrams to support your answer. Refraction through a lens, Concise Physics Class 10 Solutions.

Concave Lens

What are the three principal rays that are drawn to construct the ray diagram for the image formed by a lens? Draw diagrams to support your answer. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 2

In the diagrams below, XX’ represents the principal axis, O the optical centre and F the focus of the lens. Complete the path of the rays A and B as they emerge out of the lens.

(a)

In the diagrams, XX' represents the principal axis, O the optical centre and F the focus of the lens. Complete the path of the rays A and B as they emerge out of the lens. Refraction through a lens, Concise Physics Class 10 Solutions.

(b)

In the diagrams, XX' represents the principal axis, O the optical centre and F the focus of the lens. Complete the path of the rays A and B as they emerge out of the lens. Refraction through a lens, Concise Physics Class 10 Solutions.

Answer

Completed diagrams showing the path of the rays A and B as they emerge out of the lens are given below:

Convex Lens

In the diagrams, XX' represents the principal axis, O the optical centre and F the focus of the lens. Complete the path of the rays A and B as they emerge out of the lens. Refraction through a lens, Concise Physics Class 10 Solutions.

Concave Lens

In the diagrams, XX' represents the principal axis, O the optical centre and F the focus of the lens. Complete the path of the rays A and B as they emerge out of the lens. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 3

Where must a point source of light be placed in front of a convex lens so as to obtain a parallel beam of light?

Answer

In order to obtain a parallel beam of light, the source of light should be placed at the first focal point i.e. the focal point on the left of the optical centre of the convex lens.

Question 4

Distinguish between a real and a virtual image.

Answer

Real Image Virtual Image
A real image is formed due to actual interaction of the rays refracted by the lens. A virtual image is formed when the rays refracted by the lens appear to move if they are produced backwards.
A real image can be obtained on a screen. A virtual image cannot be obtained on a screen.
A real image is inverted with respect to the object. A virtual image is erect with respect to the object.

Question 5

Study the diagram given below.

Name the lens LL'. What are the points O and O' called? Complete the diagram to form the image of the object AB. State the three characteristics of the image. Name a device in which this action of lens is used. Refraction through a lens, Concise Physics Class 10 Solutions.

(a) Name the lens LL’.

(b) What are the points O and O’ called?

(c) Complete the diagram to form the image of the object AB.

(d) State the three characteristics of the image.

(e) Name a device in which this action of lens is used.

Answer

(a) LL’ is a convex lens.

(b) The points O and O’ are called as first and second focal points.

(c) Completed diagram showing the formation of the image of the object AB is given below:

Name the lens LL'. What are the points O and O' called? Complete the diagram to form the image of the object AB. State the three characteristics of the image. Name a device in which this action of lens is used. Refraction through a lens, Concise Physics Class 10 Solutions.

(d) The three characteristics of the image are magnified, virtual and upright.

(e) This action of lens is used in the magnifying glass.

Question 6

Study the diagram below.

Study the diagram below. Name the lens LL'. What are the points O and O' called? Complete the diagram to form the image of the object AB. State three characteristics of the image. Refraction through a lens, Concise Physics Class 10 Solutions.

(i) Name the lens LL’.

(ii) What are the points O and O’ called?

(iii) Complete the diagram to form the image of the object AB.

(iv) State three characteristics of the image.

Answer

(i) LL’ is a concave lens.

(ii) The points O and O’ are called the second and first focal point.

(iii) Below is the completed diagram showing the image of the object AB:

Study the diagram below. Name the lens LL'. What are the points O and O' called? Complete the diagram to form the image of the object AB. State three characteristics of the image. Refraction through a lens, Concise Physics Class 10 Solutions.

(iv) The three characteristics of the image are virtual, erect and diminished.

Question 7

The following diagram in figure shows an object AB and a converging lens L with foci F1 and F2.

(a) Draw two rays from the object AB and complete the diagram to locate the position of the image CD. Also mark on the diagram the position of eye from where the image can be viewed.

The diagram in figure shows an object AB and a converging lens L with foci F1 and F2. Draw two rays from the object AB and complete the diagram to locate the position of the image CD. Also mark on the diagram the position of eye from where the image can be viewed. State three characteristics of the image in relation to the object. Refraction through a lens, Concise Physics Class 10 Solutions.

(b) State three characteristics of the image in relation to the object.

Answer

(a) Below is the completed diagram showing the position of the image CD and the position of eye from where the image can be viewed:

The diagram in figure shows an object AB and a converging lens L with foci F1 and F2. Draw two rays from the object AB and complete the diagram to locate the position of the image CD. Also mark on the diagram the position of eye from where the image can be viewed. State three characteristics of the image in relation to the object. Refraction through a lens, Concise Physics Class 10 Solutions.

(b) The three characteristics of the image are magnified, virtual and upright.

Question 8

The diagram given below in figure shows the position of an object OA in relation to a converging lens L whose foci are at F1 and F2.

The diagram given below in figure shows the position of an object OA in relation to a converging lens L whose foci are at F1 and F2. Draw two rays to locate the position of the image. State the position of the image with reference to the lens. Describe the three characteristics of the image. Describe how the distance of the image from the lens and its size change as the object is moved towards F1. Refraction through a lens, Concise Physics Class 10 Solutions.

(i) Draw two rays to locate the position of the image.

(ii) State the position of the image with reference to the lens.

(iii) Describe the three characteristics of the image.

(iv) Describe how the distance of the image from the lens and its size change as the object is moved towards F1.

Answer

(i) Below ray diagram shows the position of the image:

The diagram given below in figure shows the position of an object OA in relation to a converging lens L whose foci are at F1 and F2. Draw two rays to locate the position of the image. State the position of the image with reference to the lens. Describe the three characteristics of the image. Describe how the distance of the image from the lens and its size change as the object is moved towards F1. Refraction through a lens, Concise Physics Class 10 Solutions.

(ii) Position of images will be more than twice the focal length of the lens.

(iii) The three characteristics of this image are magnified, real and inverted.

(iv) When the object move towards F1, the image will shift away from F2 and it will be magnified.

The image will form at infinity and it is highly magnified at F1.

Between F1 and optical centre, the image will form on the same side of the object and will be magnified.

Question 9

A converging lens forms the image of an object placed in front of it, beyond 2F2 of the lens.

(a) Where is the object placed?

(b) Draw a ray diagram to show the formation of an image.

(c) State three characteristics of the image.

Answer

(a) The object is placed between F1 and 2F1.

(b) Below is the ray diagram showing an object placed between F1 and 2F1 of the lens and its image is formed beyond 2F2 of the lens:

A converging lens forms the image of an object placed in front of it, beyond 2F2 of the lens. Where is the object placed? Draw a ray diagram to show the formation of an image. State three characteristics of the image. Refraction through a lens, Concise Physics Class 10 Solutions.

(c) The three characteristics of the image is real, inverted and magnified.

Question 10

A convex lens forms an image of an object equal to the size of the object.

(a) Where is the object placed in front of the lens?

(b) Draw a diagram to illustrate it.

(c) State two more characteristics of the image.

Answer

(a) The object is placed at a distance equal to twice the focal length of the lens (i.e., Object is placed at 2F1).

(b) Below diagram illustrates the formation of an image equal in size of the object by a convex lens:

A convex lens forms an image of an object equal to the size of the object. Where is the object placed in front of the lens? Draw a diagram to illustrate it. State two more characteristics of the image. Refraction through a lens, Concise Physics Class 10 Solutions.

(c) The image formed is real and inverted

Question 11

A lens forms an erect, magnified and virtual image of an object.

(a) Name the kind of lens.

(b) Where is the object placed in relation to the lens?

(c) Draw a ray diagram to show the formation of image.

(d) Name the device which uses this principle.

Answer

(a) When a lens forms an erect, magnified and virtual image of an object then we can say that it is a convex lens.

(b) The object is placed between the lens and focus (F1)

(c) Below ray diagram shows the formation of the image:

A lens forms an erect, magnified and virtual image of an object. Name the kind of lens. Where is the object placed in relation to the lens? Draw a ray diagram to show the formation of image. Name the device which uses this principle. Refraction through a lens, Concise Physics Class 10 Solutions.

(d) This principle is used in magnifying glass.

Question 12

A lens forms an image between the object and the lens.

(a) Name the lens.

(b) Draw a ray diagram to show the formation of such an image.

(c) State three characteristics of the image.

A lens forms an image between the object and the lens. Name the lens. Draw a ray diagram to show the formation of such an image. State three characteristics of the image.

Answer

(a) When a lens forms an image between the object and the lens then we can say that it is a concave lens.

(b) Below ray diagram shows the formation of the image:

A lens forms an image between the object and the lens. Name the lens. Draw a ray diagram to show the formation of such an image. State three characteristics of the image. Refraction through a lens, Concise Physics Class 10 Solutions.

(c) The three characteristics of the image are virtual, erect and diminished.

Question 13

Classify as real or virtual, the image of a candle flame formed on a screen by a convex lens. Draw a ray diagram to illustrate how an image is formed.

Answer

When the candle is placed beyond 2F1, the image is formed between F2 and 2F2.

The image is real, diminished and inverted.

In the ray diagram shown below, the candle is represented as AB and its real, inverted and diminished image is represented as A’B’ formed between F2 and 2F2.

Classify as real or virtual, the image of a candle flame formed on a screen by a convex lens. Draw a ray diagram to illustrate how an image is formed. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 14

Show by a ray diagram that a diverging lens cannot form a real image of an object placed anywhere on its principal axis.

Answer

Show by a ray diagram that a diverging lens cannot form a real image of an object placed anywhere on its principal axis. Refraction through a lens, Concise Physics Class 10 Solutions.

When the object is at any finite distance from the concave lens, the image formed is virtual, erect and diminished. Hence, a diverging lens (concave lens) cannot form a real image.

Question 15

Draw a ray diagram to show how a converging lens can form a real and enlarged image of an object.

Answer

Draw a ray diagram to show how a converging lens can form a real and enlarged image of an object. Refraction through a lens, Concise Physics Class 10 Solutions.

When the object is placed between 2F1 and F1 then the image is formed beyond 2F2.

The image is real, enlarged and inverted.

Question 16

A lens forms an upright and diminished image of an object placed at its focal point. Name the lens and draw a ray diagram to show the formation of an image.

Answer

It is a concave lens.

Below ray diagram shows the formation of an image by a concave lens:

A lens forms an upright and diminished image of an object placed at its focal point. Name the lens and draw a ray diagram to show the formation of an image. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 17

Draw a ray diagram to show how a converging lens is used as a magnifying glass to observe a small object. Mark on your diagram the foci of the lens and the position of the eye.

Answer

Draw a ray diagram to show how a converging lens is used as a magnifying glass to observe a small object. Mark on your diagram the foci of the lens and the position of the eye. Refraction through a lens, Concise Physics Class 10 Solutions.

When the object is placed between the convex lens and focal point F1 then the image is formed on the same side of the lens. The image so formed is enlarged.

As the image formed is enlarged so we can say that the converging lens can be used as a magnifier.

Question 18

Draw a ray diagram to show how a converging lens can form an image of the Sun. Hence give a reason for the term ‘burning glass’ for a converging lens used in this manner.

Answer

Below ray diagram shows how a converging lens can form an image of the Sun:

Draw a ray diagram to show how a converging lens can form an image of the Sun. Hence give a reason for the term burning glass for a converging lens used in this manner. Refraction through a lens, Concise Physics Class 10 Solutions.

The Sun is at infinity so convex lens forms its image at the second focal point which is real and very much diminished in size.

The rays of light from the sun converge to a single point called the focus of the lens. This concentrated image of the Sun burns the paper kept below the lens. Hence, the term burning glass is used for the lens.

Question 19

A lens forms an inverted image of an object.

(a) Name the kind of lens.

(b) State the nature of the image whether real or virtual?

Answer

(a) It is a convex lens as a convex lens forms an inverted image when the object is placed anywhere from infinity to F1.

(b) The image formed by a convex lens is real.

Question 20

A lens forms an upright and magnified image of an object.

(a) Name the lens.

(b) Draw a labelled ray diagram to show the image formation.

Answer

(a) The lens is a convex lens. A convex lens forms an upright and magnified image, when the position of the object is between the lens and F1.

(b) Below labelled ray diagram shows the image formation:

Draw a ray diagram to show how a converging lens is used as a magnifying glass to observe a small object. Mark on your diagram the foci of the lens and the position of the eye. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 21

(a) Name the lens which always forms an erect and virtual image.

(b) State whether the image in part (a) is magnified or diminished?

Answer

(a) A concave lens always forms an erect and virtual image.

(b) The image formed by a concave lens is always diminished.

Question 22

Can a concave lens form an image of size two times that of the object? Give reason?

Answer

No, a concave lens cannot form an image of size of two times that of the object as a concave lens diverges the rays incident on it, and the image is always diminished.

Question 23

Give two characteristics of the image formed by a concave lens.

Answer

A concave lens diverges the rays incident on it, and so the image formed by a concave lens is always virtual and diminished.

Question 24

Give two characteristics of the virtual image formed by a convex lens.

Answer

The virtual image formed by a convex lens will be erect and magnified.

Question 25

In each of the following cases, where must an object be placed in front of a convex lens so that the image formed is,

(a) at infinity,

(b) of same size as the object,

(c) inverted and enlarged,

(d) upright and enlarged?

Answer

(a) In order to form the image at infinity, the object should be placed at focus, in front of a convex lens.

(b) In order to form the image of same size as the object, the object should be placed at 2F, in front of a convex lens.

(c) In order to form an inverted and enlarged image, the object should be placed between F and 2F, in front of a convex lens.

(d) In order to form an upright and enlarged image, the object should be placed between optical centre and focus, in front of a convex lens.

Question 26

Complete the following table —

Type of lens Position of object Nature of image Size of image
Convex Between the optical centre and focus
Convex At focus
Concave At infinity
Concave At any distance

Answer

Type of lens Position of object Nature of image Size of image
Convex Between the optical centre and focus Virtual and upright Magnified
Convex At focus Real and inverted Highly magnified
Concave At infinity Virtual and upright Highly diminished
Concave At any distance Virtual and upright Diminished

Question 27

State the changes in the position, size and nature of the image when the object is brought from infinity up to the convex lens. Illustrate your answer by drawing the ray diagrams.

Answer

(i) When the object is situated at infinity, the position of the image is at F2. The image is real and inverted. It is highly diminished in size.

State the changes in the position, size and nature of the image when the object is brought from infinity up to the convex lens. Illustrate your answer by drawing the ray diagrams. Refraction through a lens, Concise Physics Class 10 Solutions.

(ii) When the object is situated beyond 2F1, the position of the image is between F2 and 2F2. The image is real and inverted. It is diminished in size.

State the changes in the position, size and nature of the image when the object is brought from infinity up to the convex lens. Illustrate your answer by drawing the ray diagrams. Refraction through a lens, Concise Physics Class 10 Solutions.

(iii) When the object is situated at 2F1, the position of the image is at 2F2. The image is real and inverted. It is of the same size as the object.

State the changes in the position, size and nature of the image when the object is brought from infinity up to the convex lens. Illustrate your answer by drawing the ray diagrams. Refraction through a lens, Concise Physics Class 10 Solutions.

(iv) When the object is situated between 2F1 and F1, the position of the image is beyond 2F2. The image is real and inverted. It is magnified in size.

State the changes in the position, size and nature of the image when the object is brought from infinity up to the convex lens. Illustrate your answer by drawing the ray diagrams. Refraction through a lens, Concise Physics Class 10 Solutions.

(v) When the object is situated at F1, the position of the image is at infinity. The image is real and inverted. It is highly magnified in size.

State the changes in the position, size and nature of the image when the object is brought from infinity up to the convex lens. Illustrate your answer by drawing the ray diagrams. Refraction through a lens, Concise Physics Class 10 Solutions.

(vi) When the object is situated between lens and F1, the position of the image is on the same side . The image is virtual and upright. It is magnified in size.

State the changes in the position, size and nature of the image when the object is brought from infinity up to the convex lens. Illustrate your answer by drawing the ray diagrams. Refraction through a lens, Concise Physics Class 10 Solutions.

Question 3

A concave lens forms the image of an object which is:

  1. virtual, inverted and diminished
  2. virtual, upright and diminished ✓
  3. virtual, inverted and enlarged
  4. virtual, upright and enlarged

Answer

Irrespective of the position of the object, a concave lens always forms an image which is virtual, upright and diminished and it is situated on the side of the object between the focus and the lens.

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