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Home Physics Class 12 Physics To find the focal length of a convex mirror using a convex lens
Class 12 Physics Lab Experiments

To find the focal length of a convex mirror using a convex lens



Aim

To find the focal length of a convex mirror using a convex lens.

Apparatus

An optical bench with four uprights (2 fixed upright in middle two outer uprights with lateral movement), convex lens, convex mirror, a lens holder, a mirror holder, 2 optical needles (one thin, one thick), a knitting needle, a half meter scale.

Ray Diagram

Formulae Used:

Focal length of a convex mirror ƒ = R/2

Where R is radius of curvature of the mirror.

Observation:

Actual length of knitting needle, x = 15 cm.

Observed distance between image needle I and back of convex mirror, y = 15 cm

Index error = y - x = 15 – 15 = 0 cm No index correction

Observation Table:

S.No. Position of:/Object needle 0 (cm)/Lens Lcm/Mirror M cm/Image needle I (cm) Radius of Curvature MI (cm)
1 25 50 56 70.5 14.5
2 28.5 50 60 73.3 13.3
3 31.5 50 65 78.4 13.4
4 30.5 50 60 74 14

Mean R = 13.8

Calculations:

Mean corrected MI = R = 13.8 cm ƒ= R/2 = 6.9cm

Result:

The focal length of the given convex mirror = 6.9 cm

Precautions:

  1. The tip of the needle, centre of the mirror & centre of lens should be at the same height.
  2. Convex lens should be of large focal length.
  3. For one set of observations, when the parallax has been removed for convex lens alone, the position of the lens & needle uprights should not be changed.











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