Finding the Focal Length of a Convex Lens

Aim

To find the image distance for varying object distances in case of a convex lens and to find its focal length.

Materials Required

Procedure

1. Mount the convex lens at the centre of the optical bench (at 50 cm mark).
2. Place the candle on one side of the lens, far away from it (beyond 2F).
3. Place the screen on the other side of the lens.
4. Adjust the position of the screen until a sharp, clear inverted image of the candle flame is obtained.
5. Record the object distance (u) and image distance (v).
6. Repeat the experiment for different object positions.
7. Calculate the focal length using the lens formula: 1/f = 1/v - 1/u for each reading.
8. Find the average focal length.

Precautions

• The principal axis of the lens, the tip of the candle flame, and the centre of the screen must be at the same height.
• The object should be placed beyond the focal length to get a real image on the screen.
• The lens should be clean.
• All distances should be measured from the optical centre of the lens.