Class 12 Physics Lab Experiments

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

May 28 2023

Aim

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

between μ and v between 1/μ and 1/v

Apparatus

An optical bench with four uprights, a convex lens (less focal length), a concave lens (more focal length), two lens holder, two optical needles, a knitting needle & a half – metre scale.

Formulae Used:

From lens formula, we have:

ƒ = μv/μ-v

Observation:

Actual length of knitting needle, x= 15 cm.

Observed distance between object needle & the lens when knitting needle is placed between them, y = 15 cm.

Observed distance between image needle & the lens when knitting needle is placed between them, z = 15 cm.

Index correction for μ = x – y = 0 cm

Index correction for v = x – z = 0 cm

Observation Table:

S.No.	Position of cm/0 (cm)/L ₁at O ₁/I/L ₂/I’	μ=IL ₂	v= I’L₂	>ƒ =	ƒ=μv/μ-v
1	29	50	75	69	78	6.0	9.0	–18.0
2	27	50	71.5	65	77.5	6.5	12.5	–13.54
3	25	50	70.5	65	72.8	5.5	7.8	–18.64
4	28	50	71.3	63	71.2	8.3	8.2	–17.45

Calculations:

Mean ƒ = ƒ₁ + ƒ₂ + ƒ₃ + ƒ ₄/4

= – 16.9 cm ≈ -17cm.

AB = AC = 2 ƒ or OC = OB = 2ƒ

Result:

The focal length of given concave lens = – 17 cm.

From 1/μ-1/v graph is, ƒ = 10.2 cm

Precautions:

The lenses must be clean.
A bright image should be formed by lens combination.
Focal length of the convex lens should be less than the focal length of the concave lens, so that the combination is convex.