Laboratory work n 63
Investigation phenomena of polarization of light
THE AIM: to study the phenomena of polarization of light, checking Malus’ law.
INSTRUMENTATION AND APPLIANCES : laser, analyzer, mirror, photocell, microammeter.
The short theory
Light is a transverse wave. The direction of the vibrating electric and magnetic vectors are at right angles to the direction of propagation ( Fig. 1 ).
Figure 1
Light is
emitted by a great number of atoms. The plane of oscillations of
vector is not the same in space. The orientations of this vector as
well as vector
are arbitrary ( Fig. 2a ).
This is so called a natural or unpolarized light. If there is an interaction of light and substance the effect of polarization takes place.
A plane
polarized wave is shown in Fig.2b. The plane of oscillations of
vector is regulated. There are three ways of producing a polarized
light: reflection of light; refraction of light; transmission of
light through anisotropic substance ( calcite crystal CaCO3
).
According to the law of Malus, the intensity of light transmitting through the analyzer can be calculated by the next formula:
,
where
is a transmitted intensity ;
is the maximum value of the transient intensity, it is equal
to the intensity of light, falling on the analyzer;
is the
angle between the plane of polarizer and the plane of
analyzer.
Experimental part
The device for investigation of the phenomena of light polarization is shown in the Fig. 4.
Figure 4
Laser is the source of plane polarized light;
mirror is used for changing the direction of light propagation;
analyzer is polaroid used to study the polarized state of light;
photocell is registrated the intensity of the light, being transmitted through the analyzer;
ammeter is used to measure current intensity which appears in the photocell under the light effect.
It is necessary to rotate the analyzer and measure the current intensity, being shown by the ammeter. The current intensity is directly proportional to the light intensity.
The order of operations
Switch on the Laser.
Rotate the analyzer and obtain maximum value of light intensity ( this corresponds to maximum value of the current intensity ). As to the law of Malus, angle
in this case is equal to 0
.Rotate the analyzer from this position to 10
(
from 0
to 90
)
and write down the results of the measurements (
and
)
in to the table.
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0 |
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10 |
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20 |
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30 |
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40 |
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50 |
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60 |
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70 |
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80 |
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90 |
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,
mА
Switch off the Laser.
Plot the graph of relationship of
to
according to the results of your measurements ( the method of
linearization is used ).Make up the conclusion on the validity of the law of Malus under the conditions of your experiment.