Page 73 - Physics - XII

4. Principal axis of the lens should be parallel to the scale of the optical bench.
5. The object needle must be placed at such distance so that only real, inverted image of it is formed.

6. The index correction for u and v should be applied.

Sources of Error

1. The uprights might be shaky.
2. Parallax removal may not be perfect.

EXPERIMENT - 4

Aim
To fi nd the focal length of a concave lens, using a convex lens.

Apparatus and Materials Required

A given concave lens, a convex lens of focal length smaller than that of concave lens, two lens-holders,
an optical bench with four uprights, two needles, an index needle (knitting needle), and a metre scale

L
1

O I
O
1
Object needle Image needle

(a)

L 1 L 2

O
O O I'
1 2 I

(b)
u
v
Fig. 4.1 (a) and (b): Determination of focal length of concave lens using convex lens

Principle/Theory

We know that a concave lens always forms a virtual image, its focal length cannot be determined directly as
in case of convex lens. Therefore, in order to determine the focal length of a concave lens, we use indirect
method, which is described below:
Let I be a real image of an object O formed by a convex lens L as shown in Fig. 4.1 (a). When a concave
1
lens L is inserted between lens L and image I, the real image of O is now formed at I' as shown in Fig. 4.1
2
1
(b). For concave lens I acts as virtual object and I' acts as real image of virtual object I formed by lens L .
2
Hence, O I = u and O I' = v. Using lens formula, we can calculate the focal length f of concave lens L as,
2 2 2
1 1 1 f uv
f v u or uv

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