CBSE Class 12 Physics Practical Section-B Exp – 3

Experiment -3

Objective

Find the focal length of convex lens by plotting a graph between u and v.

Formula Used

Where,

f = focal length of convex lens.

u = distance of object needle from optical centre of lens.

v = distance of image needle from optical centre of lens.

Note:- According to sign convention u has -ve value and v has +ve value, hence f comes +ve .

Ray Diagram

Rough focal length =20 cm

Observation table

Result

From graph, focal length of convex lens is 20cm.

Precautions

1. The tips of the needles and the center of the lens should be at the same height.
2. Parallax must be removed tip to tip.
3. The index correction should be taken into account.
4. The uprights carrying the lens and the needles should not be shaky.

Sources of error

1. The needles are not at the same height.
2. Index correction not properly applied.
3. Parallax not removed.