Page 90 - Physics - XII

3. The lens and plane mirror should be cleaned before use.
4. The screw of the spherometer should be moved in one direction to avoid backlash error.

Sources of Error
1. The distances measured with the help of plumb line may not have been measured correctly.
2. The parallax may not have been removed tip to tip.

3. The backlash error may not have been avoided.

EXPERIMENT - 8

Aim
To fi nd the refractive index of a liquid using a concave mirror and a plane mirror.

Apparatus and Materials Required

A concave mirror, water, an optical needle, a clamp stand, one metre scale, and plumb line.

Principle/Theory
When the tip of an object needle is at the C Image
centre of curvature C, the tip of image will Object needle
exactly coincide with it (Principal axis is

vertical to the plane). When water is fi lled r C′
in concave mirror, object needle is shifted at
C' to remove parallax between tips of object
needle and its image. Clamp stand i

A ray starting from C will reach at E without N
deviation because it is along radius of i
curvature. Water
D B
Due to water in the concave mirror, the
position of object and image shifts to C', M P
i.e. now ray starting from C' after refraction E
moves along ED and then DC to make Fig. 8.1: Refractive index of liquid
apparent image of centre of curvature.

∴ ∠NDC' = i (Angle of incidence) = ∠BC'D

∠MDE = r (Angle of refraction) = ∠BCD
sini DB / DC' DC
∴ µ = = =
a w sin r DB / DC DC'
DB
In ∆BC'D, sin i =
DC'
DB
and ∆BCD, sin r =
DC

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