## CBSE Class 10 Science Practical Skills – Image Formation by a Convex Lens

BASIC BUILDING CONCEPTS
Students are advised to go through the basic concept of experiment no. 4(b) before going through this.
Image
An optically representative reproduction of an object, formed by a lens or mirror due to the phenomenon of reflection or refraction is called the image of an object.

Types of Image:

1. Real image and
2. Virtual image

Real Image

• It is formed by the light rays after reflection or refraction when they
• actually meet or intersect with each other, or
• actually converge at a point.
• It can be obtained on the screen.
• It is always inverted, i.e. upside down with respect to the object.
• The size of the real image depends on the position of the object. So, it can be diminished, or of same size as that of the object or enlarged.
• It is formed by both convex lens and concave mirror.

Virtual Image

• It is formed by the light rays after reflection or refraction when they
• appear to diverge from a point, or
• appear to meet when they are produced in the backward direction, or
• intersect apparently.
• It cannot be trapped on the screen.
• It is always erect, i.e. upside up with respect to the object.
• It is formed by both concave and convex lens. It is also formed by concave, convex and plane mirrors.

New Cartesian Sign Convention
By using New Cartesian Sign Convention, one can study the formation of images by convex lens as well as concave lens also.

To understand the image formation by a convex lens, we consider the convex lens as a thin lens having a small aperture which is much less than its radius of curvature. Let its optical centre be considered as origin and its principal axis as the x-axis of the co-ordinate system.
In order to draw ray diagram for the lenses, following rules will be taken into consideration:

1. The object is always kept on the left of the lens, i.e. the light from the object is incident on the lens from the left hand side of the lens.
2. All the distances parallel to the principal axis will be measured from the optical centre of the lens.
3. All the distances are measured along the positive x-axis, (i.e. in the direction of incident ray) will be taken as positive.
4. All the distances are measured along the negative x-axis (i.e. in the direction opposite to that of incident ray) will be taken as negative.
5. All the distances measured above the principal axis (i.e. along the positive y-axis) will be taken as positive.
6. All the distances measured below the principal axis (i.e. along the negative y-axis) will be taken as negative.

The New Sign Convention for the convex lens can be illustrated as follows: Image Formation by a Convex Lens
With the help of the mentioned sign conventions, one can locate the image of the object formed by a convex lens.
The following points must be considered for drawing the ray diagrams:

1. A ray of light from the object placed on the principal axis on the left hand side of the lens, parallel to the principal axis, will pass through the principal focus F2 after refraction through the lens as shown in figure (a).
2. A ray of light passing through the first principal focus Fx will emerge parallel to the principal axis after refraction from a convex lens as shown in the figure (b).
3. A ray of light passing through the optical centre ‘O’ of the lens, emerges undeviated, i.e. it does not suffer any deviation from its path after refraction as shown in the figure (c). Using any of the two given rules and New Cartesian Sign Convention, neat ray diagrams can be drawn for the various positions of the object in front of a convex lens in order to locate the position of the image, its nature and the size.

Magnification Produced by a Lens
It is the ratio of the size of image (h2) to the size of object (h1). It is represented by ‘m’
m = Size of image(A’B’) / Size of the object (AB) = h2 / h1
According to the New Cartesian Sign Convention,

• it is negative when real image is formed by a convex lens.
• it is positive when virtual image is formed by a convex or concave lens.

EXPERIMENT – (a)

AIM
To find the image distance for varying object distances in case of a convex lens and draw corresponding ray diagrams to show the nature of image formed.

MATERIALS REQUIRED
A thin convex lens, a lens holder fitted in a stand with centered mark, a piece of a semi-transparent sheet act as screen fixed to a stand with centered mark, a small candle and a stand for candle with centered mark and a meter scale (or a ruler)

THEORY
The position, nature and size of the image of an object formed by a thin convex lens depends on the position of the object with respect to its optical centre and can be studied by using New Cartesian Sign Convention and drawing the corresponding ray diagrams.
A real, inverted image of an object can be focussed on a screen placed on the other side of the convex lens if the object is placed beyond the focus of the convex lens as shown below. The nature, position and size of the image can be noted and measured from the optical centre of the convex lens.

PROCEDURE

1. Fix a thin convex lens on a lens holder and place it vertically on the table.
2. Place the semi-transparent paper screen in a vertical position fitted to a stand on the right hand side of the convex lens.
3. Mount the candle in a centered mark stand.
4. Set-up the arrangement as shown above.
5. Place the lighted candle stand at a distance far off (or infinite distance) beyond 2F1 vertically in front of a thin convex lens. Adjust the height of the centre of convex lens nearly equal to the height of the flame of the candle as shown below. Here the flame is considered as the object AB. Measure and record the height h of the candle flame when the flame does not flicker.
6. Obtain the sharp, real image of the candle flame on the screen by moving it in the forward or backward direction. With the help of measuring scale, record the position of lens L and screen S in the observation table. The difference between these two readings (the centre of the convex lens and centre of the screen) gives the approximate focal length of the convex lens.
7. Now shift the lighted candle nearby but beyond twice the approximate focal length (2F) of the thin convex lens.
8. Adjust the position of the screen by moving it away from the previous position to get a sharp and real image A’B’ of the candle flame AB on it. Note and record the position of the lighted candle AB, screen s and lens l. Also measure and record the height h’ of the image of the flame of the lighted candle obtained on the screen.
9. Repeat the experiment by placing the lighted candle in front of the convex lens LL’ at a distance
• equal to 2F
• less than 2F but more than F.
10. Locate the position of screen in each case by moving it still further away from lens to get the real and sharp image of the flame and record your observations for the position and height of image in the observation table in each case.
11. Now keep the lighted candle at F and try to locate the image on the screen by moving it gradually away from the lens. You may not be able to obtain the image because when the object is at F, the image will be formed at infinity and is blurred due to high magnification.
12. Find the distance between the optical centre of the lens and candle flame (object) x (say) and corresponding image distance between the optical centre O of the lens and the screen, y (say).
13. Repeat the experiment three more times by varying x by changing the position of the lens. Locate the sharp image of the flame and record the position and height of the image in each case.
14. With the help of data obtained, draw the corresponding ray diagrams in each case to show the nature of image formed.

OBSERVATIONS AND CALCULATIONS
Approximate focal length of the thin convex lens, F = …………. cm
Height of the candle flame, h = ………….. cm. RESULT

1. As the object is moved from infinity towards the optical centre of the convex lens,
• the image distance increases gradually and
• size of image also increases gradually.
2. When the object is at infinite distance, the image formed by the convex lens is real, inverted and sharp but diminished in size at the focus is obtained. This gives the approximate focal length of the convex lens. The corresponding ray diagram is shown below. 3. When the position of object is beyond 2F1, the image formed is real, inverted, and smaller in size and lies in between F2 and 2F2. The corresponding ray diagram is shown in figure (b) in experiment 1(b).
4. When the position of object is at 2F1, the image formed is real, inverted, same size as the size of object and lies at 2F2. The corresponding ray diagram is shown in figure (c) in experiment 1(b).
5. When the position of object lies between F1 and 2F1 the image formed is beyond 2F2. It is real, inverted and enlarged or magnified. The corresponding ray diagram is shown in figure (d) in experiment 1(b ).
6. When the position of object lies at F1, the image formed is at infinity, It is real, inverted and highly enlarged. The corresponding ray diagram is shown in figure (e) in experiment 1(b).

PRECAUTIONS

1. The convex lens should be thin and having a small aperture and should be without any scratches to get the distinct image of the lighted candle.
2. The centre of the lens, the tip of the candle flame and the centre of the screen should lie in the same straight line and parallel to the measuring scale.
3. The base of the candle stand, convex lens and screen should be parallel to the measuring scale.
4. All distance should be measured between the centred mark of each stand.
5. The focal length of the thin convex lens must preferably be between 15 to 20 cm.
6. The convex lens should be kept in such a way that the light rays coming from the lighted candle always falls on its aperture without any obstruction.
7. Flame must be uniform throughout the experiment. To avoid its flickering, switch off the fan and perform the experiment in calm air. Otherwise you can use the long filament bulb instead of burning candle if the tip of its flame is not found to be stable.
8. It is better to perform this experiment in a dark room or in shade where no direct light reaches to the working table to get the sharp and distinct images of the candle flame.
Note: It is advisable for the students that after doing the candle screen experiment for measuring the image distance, they must perform the following activity to understand how to draw the images of an object formed by a convex lens for the various positions of object.

EXPERIMENT – (b)

AIM
To draw the images of an object formed by a convex lens when placed at various positions.

MATERIALS REQUIRED
A drawing board, sheets of white paper, measuring scale, protractor and drawing pins or adhesive tape.

THEORY
The New Cartesian Sign Convention and image formation rules for the convex lens help us to draw ray
diagrams forming images by a convex lens for various positions of the object.
The position of the object may be

• at infinity,
• beyond 2F1
• at 2F1,
• between F1 and 2F1
• at F1
• between focus (F1) and optical centre (O) of the convex lens.

For the sake of convenience and clarity of the ray diagram, two rays are considered. The intersection of two refracted rays gives the position of the image formed by the lens.

PROCEDURE

1. Fix a white sheet of paper on a drawing board with the help of drawing pins.
2. Draw a thin line 12-15 cm in length in the middle of the paper. Name it as XX’.
3. Mark point ‘O’ at the centre of the line. Draw a perpendicular line of equal height at the point O above and below XX’. Name it L1L2.
4. Make a thin convex lens of small aperture around L1L2. Assume O as optical centre of the lens.
5. Mark points F1 and F2 on the line XX’ on either side of the lens L1L2 such that OF1 = OF2 where F1 and F2 are the two principal foci of the lens.
6. Also mark points 2F1 and 2F2 on the line XX’ such that O(2F1) = 2(OF1) and O(2F2) = 2(OF2).
OBJECT AT INFINITY
7. Draw an object AB of suitable height ‘h{ at a very far distance from the lens considered to be placed at infinity.
8. Draw the lines parallel to the principal axis such as CD, GH, PQ and RS incident on the surface of convex lens.
9. Draw the refracted emergent rays on the other side of the lens such as DF2, HF2, QF2 and SF2 through the convex lens and intersect at the principal focus F2. A diminished image of the distant object is formed at F2. It is real and inverted. 10. Record the result in an observation table.
OBJECT BEYOND 2F1
11. Fix another white sheet of paper on the drawing board.
12. Repeat the steps (2) to (6).
13. Draw an object AB of suitable height ‘h1‘ beyond 2F1 as shown.
14. Draw a ray AE parallel to the principal axis F1OF2 incident on the surface of convex lens at point E.
15. Draw another ray AO through the optical centre O of a convex lens and extend it to other side of the lens as OA’.
16. On the other side of a lens, draw the ray EF2 passing through the focus F2 and interszecting the ray OA’ at A’ as shown in the figure, forming an image B’A’. 17. Measure the height of the object AB(h1) and height of the image A’B'(h2).
18. We observe from the figure that the image formed is real, inverted, smaller in size and in between F2 and 2F2.
19. Record these observations in the observation table.
FOR OTHER POSITIONS OF THE OBJECT
20. Repeat the above steps similar to the previous case. Draw neat ray diagrams for other positions of the object as shown in the,
Figure (c) at 2F1.
Figure (d) between F1 and 2F1.
Figure (e) at focus F1.
Figure (f) is in between focus F1 and optical centre O of the convex leris. 21. Measure the height of the object AB(h1) and height of image A’B'(h2) respective in all cases (c) to (f) as listed in step 20. Record them in the observation table.
22. Note down the nature, relative size and position of the image formed by the convex lens for the various positions of the object.
23. Tabulate your observations in the observation table.

OBSERVATION AND CALCULATION RESULT
The characteristics of image formed by a convex lens for various positions of the object is summarised in the observation table.

PRECAUTIONS

1. Use very sharp tipped pencil to draw thin lines in various ray diagrams.
2. The convex lens drawn should be thin and of small aperture for obtaining the distinct image.
3. Protractor should be used to draw the convex lens.
4. All uprights should be vertical.
5. The aperture of the lens in all figures should be same.
6. Proper arrows should be drawn to indicate the incident and the refracted rays.

INTERACTIVE SESSION

Question 1.
What is double convex lens?
A lens which has two refracting spherical surfaces, bulging outwards is called a double convex lens or simply convex lens.

Question 2.
What is the function of a convex lens?
It can converge the incident light rays.

Question 3.
Which law is obeyed by the light rays when refracted through a convex lens?
Laws of refraction.

Question 4.
Name the point on the principal axis of a convex lens where the ray of light passing through it does not suffer any deviation.
Optical centre ‘O’ of the lens.

Question 5.
What happens to the size of image when the object is moved from infinity towards the optical centre of the lens?
The size of the image increases gradually.

Question 6.
What would be the nature of image when the object lies in between infinity and F?
Real and inverted

Question 7.
Suppose you move the candle from the focus of a convex lens towards infinity. In which direction should the screen be moved to get the real and sharp image?
Screen must be shift towards the lens.

Question 8.
What happens when the experiment is performed with scratches on the lens?
Sharp and distinct image cannot be obtained accurately.

Question 9.
The image of a candle formed by a convex lens is obtained on a screen. Will the full size of image be obtained if the lower half of the lens is covered with black paper and made completely opaque ?
Yes, the full size image with less brightness will be obtained by the another half portion of the convex lens.

Question 10.
According to the New Cartesian Sign Convention, what is the nature of a focal length of a convex lens?
Positive.

Question 11.
Is virtual image erect or inverted?
Erect and enlarged.

Question 12.
Where should an object be placed in order to use a convex lens as a magnifying glass?
Between focus and optical centre of the lens.

Question 13.
On what factor does the size of an image formed by a lens depend?
It depends on the position of an object from the lens.

Question 14.
Why do you consider only two rays to draw the ray diagram?

• For the sake of clarity of ray diagram.
• To know their directions easily after refraction from the lens.

Question 15.
Why do you draw the equiconvex lens?
If the drawn lens is not equiconvex, then the condition OF1 = OF2 will not hold good.

Question 16.
According to the New Cartesian Sign Convention, what will be the sign of image distance when it is formed beyond 2F2?
Positive.

Question 17.
Which of the following quantities remains the same when light goes from one medium to another medium?

• speed
• frequency
• wavelength
• intensity Examinee

Frequency.

NCERT LAB MANUAL QUESTIONS

Question 1.
What is the nature of an image formed by a thin convex lens for a distant object? What change do you expect if the lenses were rather thick?
Nature of image formed by a thin convex lens for a distant object is real, inverted and highly diminished. It is formed at the focus of a convex lens. Thick convex lens has shorter focal length so image distance will be reduced as compared to thin lens.

Question 2.
You are provided with two convex lenses of same aperture and different thickness. Which one of them will be of shorter focal length?
Thick convex lens will have shorter focal length than that of thin convex lens.

Question 3.
How will you distinguish between a convex lens and a concave lens by holding in hand and looking the printed page through them?
If the letters of printed page appears enlarged then it is a convex lens, but if they appear diminished then it js concave lens.

Question 4.
A distinct image of the lighted candle has been obtained on screen with fixed position using a thin convex lens. Why does the image of the candle get blurred if the position of any one of them is slightly disturbed?
The image of lighted candle get blurred due to change in candle position or lens position while keeping position of screen be fixed, the rays may meet before or after the screen.

Question 5.
Why do we require a calm atmosphere to perform this experiment?
With the flickering flame, we cannot measure its height as well as its image height. Also, it would be difficult to get the sharp image other than calm atmospheric conditions.

Question 6.
Why is it preferred to perform this experiment in dark or in shade?
To get the sharp and distinct image of the candle flame, it should be better to perform this experiment in dark or in shade.

Question 7.
Sometimes, the image formed by a convex lens of an object placed at 2F1 is not of the same size and at location 2F2 on the other side of the convex lens. What could be the possible reason(s) for such a situation?
The image formed by a convex lens, when the object is placed at 2F1 is of the same size and is located at 2F2 on the other side of convex lens is true only when the convex lens is thin and having a small aperture. If the lens is thick and having a larger aperture, then there are certain defects in the image formed.

Question 8.
A ray of light is passing through the principal focus of a convex lens. How will it emerge after refraction through the lens?
It will emerge parallel to the principal axis after refraction through the convex lens because it is measured against the direction of incident ray of light.

Question 9.
An object is placed on the left side of a lens (having 10 cm focal length) at a distance of 20 cm. What will be the sign of object distance?
According to the New Cartesian Sign Convention, the sign of object distance is negative because it is measured against the direction of incident ray of light.

PRACTICAL BASED QUESTIONS
Multiple Choice Questions/VSA (1 mark)

Question 1.
Study the diagram given below: The diagram showing the correct path of the ray after refraction from the convex lens is  Question 2.
A student suggested the following guidelines to his friend for performing the experiment to draw the images of an object formed by a convex lens, when placed at various positions:
(i) A ray of light from the object, parallel to the principal axis, after refraction from a convex j lens will pass through the principal focus on the other side of the lens.
(ii) A ray of light passing through the principal focus will emerge parallel to the principal axis.
(iii) A ray of light passing through the optical centre of a lens will emerge without any deviation from the lens.
(iv) The convex lens drawn should be thick and of large aperture to obtain the clarity of ray diagram.
When he showed these guidelines to his teacher, the teacher corrected and modified one of the following guidelines:
(a) (i)
(b) (ii)
(c) (iii)
(d) (iv)

Question 3.
To draw the images of an object formed by a convex lens for its various positions, a student summarised the New Cartesian Sign Convention as given
(A) Light rays from the object falls on the lens from the left hand side.
(B) All measurements should be taken from the first principal focus of the lens.
(C) All measurements taken to the right side of optical centre are taken as positive.
(D) Distance measured from the optical centre to left side are taken as negative.
Which of the above statements in his summarisation is incorrect?
(a) A
(b) B
(c) C
(d) D

Question 4.
Identify the correct ray diagram drawn by a student Question 5.
For making the ray diagrams for a convex lens, a student must consider at least two refracted rays to locate the position of image. Out of the three rays shown by him, the incorrect one is (a) I
(b) II
(c) III
(d) I and II both

Question 6.
In order to find the image distance for varying object distance in case of convex lens, a student was asked to measure the image distance on getting the sharp image of the lighted candle placed in front of the convex lens at twice of its focal length, she should measure the distance between
(a) lens and lighted candle.
(b) tip of the lighted candle and centre of the screen.
(c) centre of lens and centre of the screen.
(d) centre of a lens and vertex of a screen.

Question 7.
The candle screen experiment gives only the quantitative idea for recording the position of lighted candle, convex lens and the screen. This is due to
(a) the height of wax candle had decreased since it was burning for long time.
(b) the readings were more or less correct but were not precise.
(c) it is not possible to place the screen exactly at the actual meeting point of the refracted rays.
(d) all of the above .

Question 8.
Study the following diagrams: [AI 2013] The diagrams showing correct path of the ray after refraction from the convex lens are
(a) I, II and III
(b) II, III and IV
(c) III, IV and I
(d) I, II and IV

Question 9.
Study the following ray diagrams drawn by four students to locate the position of the image formed by a convex lens when the object is placed at 2F: The correct ray diagram is that of student
(a) I
(b) II
(c) III
(d) IV

Question 10.
Given below are two lenses: [CBSE 2013] Teacher asked her students to draw for both the lenses, image formation for an object placed at infinity. Image distance for the two
(a) would be larger for A than for B
(b) would be smaller for A than for B
(c) would be equal for both
(d) cannot be predicted without performing experiment

Question 11.
In the diagram given below, an object is placed in front of a convex lens. The image thus formed will be [CBSE 2013]
(a) always real
(b) always erect
(c) may be real or virtual depending on its position with respect to focus (d) always virtual

Question 12.
Out of the five incident rays s find the three rays that are of refraction and may be used for locating the position of image formed by a convex lens [Delhi 2013] (a) 1, 2 and 3
(b) 2, 3 and 4
(c) 3, 4 and 5
(d) 1, 2 and 4

Question 13.
Observe the figures given below for three lenses A, B, and C Converging power of the three lenses will be respectively as
(a) A > B > C
(b) B > A > C
(c) B > C > A
(d) C > B > A

Question 14.
A student draws a ray diagram for image formation by a convex lens for object placed at 2F. According to him, the magnification produced by convex lens in this case is [CBSE 2013]
(a) +1
(b) + 2
(c) +3
(d) – 1

Question 15.
A student was asked to draw a diagram for image formation of an object placed at a distance of 4 cm from a convex lens of focal length 5 cm. After completing the diagram, he would note that the nature of image formed is [CBSE 2013]
(a) real, inverted and magnified
(b) virtual, erect and magnified
(c) virtual, inverted and magnified
(d) virtual, erect and diminished

1. (b)
2. (d)
3. (b)
4. (d)
5. (c)
6. (c)
7. (d)
8. (d)
9. (b)
10. (a)
11. (c)
12. (b)
13. (c)
14. (a)
15. (b)

Question 1.
To construct a ray diagram, we use two light rays which are so chosen that it is easy to know their directions after refraction from the convex lens. List these two rays and state the path of these rays after refraction.
Two rays chosen for refraction are:

• A ray of light parallel to the principal axis.
• A ray of light passing through optical centre of a lens.

Path of these rays after refraction:

• A ray parallel to the principal axis, after refraction through the lens, will pass through the principal focus on the other side of the lens.
• A ray of light passing through optical centre of a lens will emerge from the lens without any deviation in its path.

Question 2.
A student focused the image of a candle flame on a white screen by placing the flame at various distances from a convex lens. He noted his observations as given below: In which case

1. is the size of image smaller than the size of object?
2. is the size of object and image same?

1. When the object is placed between infinity and 2F, diminished, real and inverted image is formed. So in the first observation, size of the image is smaller than the size of object.
2. When the object is placed at 2F, an image of the same size as that of the object is formed at 2F on the other side of the lens. It is real and inverted as well. Hence in 3rd observation, the size of object and image is same.

Question 3.
Draw ray diagrams to represent the nature, position and relative size of the image formed by a convex lens for the object placed:

1. at 2F.
2. between F and the optical centre O of the lens.

1. When the object is at 2F Nature of image: Real and inverted.
Size of image: Same as that of object. Position of image: At 2F on the other side of the lens.
2. When the object is in between F and the optical centre 0 of the lens: Nature of image: Erect and virtual,
Size of image: Enlarged.
Position of image: On the same side of the lens.

Question 4.
A lighted candle, placed on a metre scale at 8 cm mark, was focused on a white screen placed at 92 cm mark, using a converging lens placed on the scale at 50 cm mark. With the help of this data, he finds that focal length of the given converging lens is 21 cm.

1. Find the position of image formed if the object is shifted towards the lens at a position of 29 cm.
2. State the nature of image formed if the object is further shifted towards the lens.

1. As object is shifted towards the lens at a position of 29 cm, new object distance is, u’ = – (50 – 29) = – 21 cm. This means that object is now placed at the focus of the convex lens. If the object is placed at the focus of a convex lens, final image will be formed at infinity as the rays coming from the focus of a convex lens emerges parallel to the principal axis.
2. If the object is further shifted towards the lens, object is now within the focus of a convex lens, the nature of image formed is
• virtual, erect, and
• enlarged or magnified, i.e. larger than the size of object.

Question 5.
It is desired to obtain the erect image of an object, using a convex lens of focal length 20 cm.

1. What should be range of object distance from the lens?
2. Will the image formed be bigger or smaller?