Page 66 - Physics - XII

where, ∆u and ∆v are the least count of the optical bench scale.

u and v are the corrected object and image distances respectively. f is the mean value of focal length.

Result
1. Mean value of focal length of concave mirror by computation from the table = _____ cm.
2. The focal length of the given concave mirror is f ± ∆ f =_____ ± _____ cm.

Precautions
1. All the uprights should be vertical and principal axis of the mirror should be horizontal.

2. Tip to tip parallax should be removed between needle and the image of the object needle.
3. Tips of the needles and the pole of the mirror should be at the same height as that of the pole of the
mirror.
4. Index correction for u and v should be properly determined and must be applied.
5. To locate the position of the image, the eye should be at least 30 cm away from the needle.
Sources of Error

1. The uprights may not be vertical.
2. The parallax removal may not be perfect.

EXPERIMENT - 2

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

Apparatus and Materials Required

An optical bench with four uprights, two needles, a convex mirror, a convex lens, an index needle, a mirror
holder, a lens holder, and a half metre scale

Principle/Theory
In fi g. 2.1(a), a convex lens L is introduced between the convex mirror M and object needle O.

M
I Image Convex mirror

O L C
P

Object
needle Convex lens
(a) R

M
O L ′ I
P

Object Image
needle (b) needle
Fig. 2.1 (a) and (b): Focal length of convex mirror using convex lens

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