Page 83 - Physics - XII

focus. The readings are recorded by means of main scale and vernier scale of high accuracy (0.01 mm)
attached to the instrument.
It has two systems of converging lenses, i.e. objective O consisting of small aperture and small focal length

and eye-piece E consisting of large aperture and large focal length. There are two separated tubes in which
the objective and eye-piece are fi tted.
V′
E

R

O

H

H′

Fig. 6.1: A travelling microscope

S S M S
V

N R M M R
V 3
r V R 2

A S B R 1
r Q
i S
P 1 i t P
P 1
D C (a) (b) (c)
P
Fig. 6.2: Real and apparent thickness Fig. 6.3: Measurement of real and apparent thickness
of the glass slab of the glass slab by a travelling microscope

Principle/Theory

On a horizontal surface, when a glass slab is placed in air and its bottom surface is viewed from top, it
appears to be elevated.
Hence, real thickness of the glass slab = PS

Apparent thickness of the glass slab = P S
1
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