AIM

To determine the focal length of a convex lens by obtaining the image of a distant object.

MATERIALS REQUIRED

A convex lens, a lens holder, a white screen such as a wall or white painted board, meter scale, distant object.

THEORY

1. A ray of light passing through the first principal focus or appearing to meet at it emerges parallel to the principal axis after refraction.
2. Convex lens converges the incident parallel beam of light on the other side of the lens.
3. T image formed by the convex lens may be real or virtual, inverted or erect, diminished, the same size or magnified depending on the various positions of the object.
4. Focal length for the convex lens is positive while it is negative for the concave lens.
5. The lens behaves like a plane glass plate when it is kept in a medium whose refractive index is equal to that of the lens.

PROCEDURE

1. Choose any distant object like a tree or the sun as an object for the experiment.

2. Held the lens vertically inside a lens holder and lens must be kept in vertical position during the experiment.

3. Place a screen on the other side of the lens.

4. Obtain the image of tree/building on a white screen or wall. Move the lens to get a sharp image.

5. Using a meter scale, measure the distance from the lens to the screen.

OBSERVATION AND CALCULATION

S. No.	The position of lens ‘L’ (cm)	The position of screen ‘M’ (cm)	Difference between screen and lens. (M – L) (cm)	Focal length f (cm)
1.	25 cm	37 cm	12 cm	F1 = 12 cm
2.	30 cm	45 cm	15 cm	F2 = 15 cm
3.	40 cm	53 cm	13 cm	F3 = 13 cm
4.	60 cm	73 cm	13 cm	F4 = 13 cm

Mean value of the focal length = 13.25 cm

RESULT

PRECAUTIONS

1. The lens should be kept vertical inside the lens holder.

2. While measuring the distance, the distance between the sharp image and the centre of the lens is to be measured.

3. The lens the screen must be at the same level.