Introduction

In this experiment, we will see image formation by a convex lens.

Image

The optical reproduction of an object after refraction or reflection through a lens or mirror is called an image of that object. There are two types of images:

1. Virtual image,
2. Real image.

Real Image

1. The image which can be obtained on a screen is called a real image.
2. It is formed at a point where light rays meet after reflection or refraction.
3. It is formed at a point where light rays converge.
4. The image formed is always inverted
5. Real images are formed by convex lenses and concave mirrors.
6. The image formed is based on the position of the object.

Virtual Image

1. The image which can not be obtained on a screen is called a virtual image.
2. It is formed at a point where light rays appear to meet after reflection or refraction.
3. It is formed at a point where light rays appear to diverge.
4. The image formed is always erect.
5. Virtual images are formed by a convex lens, concave lens, convex mirror, and concave mirror.
6. The image formed is based on the position of the object.

Sign Convention

In order to understand the sign convention of any lens, let us take a lens with a small aperture less than its radius of curvature. Take its optical center as the origin and the X-axis as the principal axis.

To understand the formation of images, we must follow the given rule.

1. The thing must be placed on the left side of the lens.
2. All distance should be measured from the origin of the lens.
3. All the gaps measured along the positive side of the X-axis from the origin must be taken positively.
4. All the space measured along the negative side of the X-axis from the origin must be taken negatively.
5. All the space measured above the principal axis is positive.
6. All the space measured below the principal axis is negative.

Image Formation by a Convex Lens

1. When a light ray is parallel to the principal axis, it passes through the principal focus F2 after refraction.

2. When a light ray passes through, the principal focus will emerge parallel to the lens after refraction.

3. When a light ray passes through the origin of the lens, it emerges un-deviated.

Magnification of a lens

The ratio of the size of the image, h2, to the size of the object, h1, is called magnification, m.

Magnification = Height of image/Height of object

m = h2/h1

1. Magnification is negative when a real image is created by a convex lens.
2. Magnification is positive when a virtual image is created by a concave lens or a convex lens.

EXPERIMENT A

Aim

To measure the image distance for varying object distance in a convex lens and draw images for the same to show the nature of the image formed.

Apparatus Required

1. A convex lens,
2. A lens holder,
3. A piece of semi-transparent sheet,
4. A candle,
5. A holder for candle,
6. A meter scale.

Theory

1. The nature, size, and position of the image depend on the position of the object placed on the left side of the lens according to the cartesian system.
2. Image distance should be measured from the origin of the lens along the positive X-axis direction.
3. A real and inverted image will be formed if the object is placed beyond the principal focus of the lens.

Procedure

1. Fix the convex lens in the holder and place it vertically.
2. Fix the white sheet of paper on the left side of the lens.
3. Mount the candle on a holder and place it on the left side of approximately 2F of the lens, as shown in the figure below.
4. Place the candle in such a way that its height is equal to the height of origin of the lense from the bottom.
5. Obtain a real and inverted image of the candle on the white sheet by moving it in a backward or forward direction.
6. Using a meter scale, record the positions of lens L and screen S in the observation table.
7. Now, you can obtain the focal length of a convex lens by subtracting these two quantities.
8. Again, place the candle a little far from the above position.
9. Move the screen a little far from the above position to obtain a sharp image of the candle flame.
10. Note the positions of the candle, screen, and lens with the help of a meter scale.

11. Also, record the height of the image obtained.
12. Repeat this experiment by placing the candle at different positions, like at 2F and between 2F and F.
13. Keep the candle at F and try to obtain the image. You will see that you are not able to obtain the image on screen because the image, in this case, will form at infinity.
14. Find the distance between the lens and the screen (y) and the distance between the lens and the object (x).
15. Repeat this experiment for different values of x.
16. Record your observation in the below table.

Observation and Calculation

Focal length of the convex lens F = ……….cm.

Height of the candle flame, H = …………cm.

Result

1. When the object moves from infinity towards the origin of the convex lens, image distance and image size increase.
2. When the object is at infinity, a real, inverted, and highly diminished image will form at the focus.
3. When the object is placed beyond 2F1, a real, inverted, and small size image will form between 2F2 and F2.
4. When the object is placed at 2F1, a real, inverted, and same size image will form at 2F2.
5. When the object is placed between 2F1 and F1, a real, inverted, enlarged image will form beyond 2F2.
6. When the object is placed at F1, a real, inverted, and highly enlarged image will form at infinity.

Precautions

1. The convex lens must be in good condition and should be thin.
2. The centre of the lens, the height of the candle flame, and the center of the screen should be placed in a straight line.
3. Protect your eyes from flame.
4. The base of all three should be placed parallel to the meter scale.
5. The focal length should be between 15cm and 20cm.
6. It will be better to perform this experiment in the absence of much light.

EXPERIMENT B

Aim

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

Apparatus Required

1. A drawing board,
2. Some sheets of white paper,
3. Drawing pins,
4. Metre scale,
5. Protector.

Theory

We obtain images from the given positions of objects.

1. At infinity,
2. Beyond 2F1,
3. At 2F1,
4. Between F1 and 2F1,
5. At F1,
6. Between F1 and the optical center.

For more clarity, two incident rays are taken.

Procedure

1. With the help of some drawing pins, fix a white sheet on the drawing board.
2. Draw a line of length 15-16cm in the centre of the paper.
3. From the center of this line, draw a perpendicular line of sem length.
4. Make a convex lens L1L2 with O as an optical centre.
5. Mark two points, F1 and F2, on either side of the lens such that OF1 = OF2.
6. Mark two points 2F1 and 2F2 on either side of the lens such that O(2F1) = 2(OF1), O(2F2) = 2(OF2) and O2F1 = O2F2.

When an Object is at Infinity

7. On the left side of the lens, draw an object AB of height h.
8. Draw some lines parallel to the principal axis. These lines are incident rays.
9. These lines converge at a point F2 after refraction. F2 is the focus of the lens.
10. A real, inverted, and enlarged image will form at F2.
11. Record your observation in a table.

Object beyond 2f1

1. Repeat steps from 1 to 6.
2. Draw an object AB beyond 2F1 of height h.
3. Draw two rays. The first one is parallel to the principal axis and the second one passes from the optical centre.
4. The point where both the rays meet is between F2 and 2F2. Here, it will form a real, inverted, and small size image.

For Other Positions of the Object

Repeat the same experiment for at 2F1, between F1 and 2F1, At F1, between F1 and optical center.

Note down the height, size, and nature of all images formed in the given table.

Precautions

1. The pencil should be sharp.
2. The lens must have a small aperture and be thin.
3. Convex lenses should be drawn by using a protector.
4. Arrows should be drawn to indicate lights.

Viva Questions With Answers

Q.1 What is an image?
ANS. The optical reproduction of an object after refraction or reflection through a lens or mirror is called an image of that object.

Q.2 Name the types of images?
ANS. There are two types of images:
1. Virtual image,
2. Real image.

Q.3 Why can we not obtain the image of an object placed at focus?
ANS. Because the image will form at infinity which is highly enlarged.

Q.4 What do you mean by Virtual image?
ANS. The Image which can not be obtained on a screen is called a virtual image.

Q.5 What do you mean by real image?
ANS. The Image which can be obtained on a screen is called a real image.

Q.6 What is the nature of an image formed when an object is kept at 2F1?
ANS. Real, inverted, and the same size as the object.

Q.7 Why is a convex lens called a converging lens?
ANS. Because it converges light rays at one point.

Q.8 Where does the image form when an object is kept at infinity?
ANS. Focus

Q.9 When virtual and erect images will form in a convex lens?
ANS. When the object is placed between focus and optical center.

Q.10 Where the object has been kept If an image forms between F2 and 2F2?
ANS. Beyond 2F2.

Vikram Sarabhai

An Indian physicist and astronomer.