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.10.1 ).

Figure 10.1

 

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. 10.2a ).

This is so called a natural or unpolarized light. If there is an interaction of light and substance the effect of polarization takes place.

Figure 10.2

 

A plane polarized wave is shown in Fig.10.2b. 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

, (10.1)

where I is a transmitted intensity ; Im is the maximum value of the transient intensity, it is equal to the intensity of light, falling on the analyzer; θ 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. 10.3.

Figure 10.3

 

1. Laser is the source of plane polarized light;

2. mirror is used for changing the direction of light propagation;

3. analyzer is polaroid used to study the polarized state of light;

4. photocell is registrated the intensity of the light, being transmitted through the analyzer;

5.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 operation

 

1. Switch on the Laser.

2. 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 θ in this case is equal to 0 .

3. Rotate the analyzer from this position to 10 ( from 0 to 90 ) and write down the results of the measurements (If and ) in to the table.

 

θ cosθ cos2θ If, mA
     
     
     
     
     
     
     
     
     
     

 

4. Switch off the Laser.

5. Plot the graph of relationship of to cos2θ according to the results of your measurements ( the method of linearization is used ).

6. Make up the conclusion on the validity of the law of Malus under the conditions of your experiment.

 

Control questions

 

1. Definition of a natural light, polarized light, plane polarized light.

2. What methods of obtaining polarized light do you know?

3. What is a polaroid, polaryzer, analyzer?

4. What is a light wave?

5. At what conditions intensity transmitted through the analyzer will be the maximum and minimum?

References

 

1. Ландcберг Г. С. Оптика. – М.: Наука, 1976.- с.370 – 379.

2. Сивухин Д. В. Общий курс физики. – т. 4. – М.: Наука, 1980.- с. 397 – 400.

3. Савельев И. В. Курс общей физики.– т. 2. – М.: Наука, 1982.- с. 428 – 432.

 

Authors: Lushchin S.P., the reader, candidate of physical and mathematical sciences.

Reviewer: Loskutov S.V., professor, doctor of physical and mathematical sciences.