03-11-2012, 10:43 AM
Speed of light in glass
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Apparatus
Prism, spectrometer, monochromatic light source, spirit level.
Objective
To determine the speed of propagation of light waves in glass.
Theory
Light travels with the speed c = 2.998 ´ 108 m s-1 in vacuum. In a material medium its
speed (v) is less. As a result, light waves undergo refraction at the interface of two media.
In this experiment, we take the material of the medium in the form of a glass prism. A
parallel stream of waves traveling from a medium 1 (here air) is incident on the interface
of air and glass (of the prism), at the angle incidence q1. The angle of refraction is q2 .
Snell’ s law connects the two by the relation,
Procedure
A spectrometer is used to measure the necessary angles. The spectrometer consists of three
units: (1) collimator, (2) telescope, and (3) prism table.
The prism table, its base and telescope can be independently moved around their common
vertical axis. A circular angular scale enables one to read angular displacements (together
with two verniers located diametrically opposite to each other).
In the experiment, we need to produce a parallel beam of rays to be incident on the prism.
This is done with the help of a collimator. The collimator has an adjustable rectangular slit
at one end and a convex lens at the other end. When the illuminated slit is located at the
focus of the lens (See Fig. 1), a parallel beam of rays emerges from the collimator. We can
test this point, with the help of a
telescope adjusted to receive parallel
rays.
Optical leveling of the Prism:
The prism table would have been nearly leveled before use have started the
experiment. However, for your experiment, you need to do a bit of leveling using
reflected rays. For this purpose, place the table with one apex at the center and facing
the collimator, with the ground (non-transparent) face perpendicular to the collimator
axis and away from collimator. Slightly adjust the prism so that the beam of light from
the collimator falls on the two reflecting faces symmetrically (Fig. 2) When you have
achieved this lock the prism table in this position. Turn the telescope to one side so as
to receive the reflected image of the slit centrally into the field of view. This may be
achieved by using one of the leveling screws.