PHYSICAL OPTICS LABORATORY
Dr.Nourhan Kawni
Dr.Salem Marhaba
Student: Roua Etry ID: 202203078
Experiment4: Malus' Law
Aim of experiment:
Verifying Malus' Law (I=I0cos2θ) relation between the intensity of light passing
before and after the analyser, and the angle between the analyser and polarizer.
Theory:
An unpolarised beam of light is polarised by passing it through a sheet of Polaroid.
The polarised beam is then passed through a second Polaroid sheet, often called the
analyzer. The transmission axis of the analyser makes an angle θ with the plane of
polarisation of the incident beam. The beam that emerges from the analyser is
polarised in the same direction as the transmission axis of the analyser.
In the first figure the beam of light passes through polarizer 1 then polarizer 2
which have angle 90° with the first. So, the beam will not reach the screen which
have a detector (voltmeter) for detecting the intensity of the light reached.
As we change the 2nd polarizer's angle the beam of light starts to pass through the
opening and thus reaching the light detector.
We will keep varying the angle θ until we reach the maximum angle which is O° with
the axis, in this case the maximum intensity of light is passing through the opening
and therefore recording the maximum voltage on the detector.
Equations:
E = E0 cos θ if we square both sides we get → E2 = E2 cos2 θ
This the equation we're going
I is proportional to E2 → I = I0 cos2 θ (Malus' Law)
to use to prove Malu's Law
2
I is proportional to U (and I0 to Umax) → U = Umax cos θ
Apparatus:
- Optical bench
- Polarizer and analyzer
- Lamp
-Connecting leads
- Voltmeter
- Voltage detector
Method:
1. Set up the apparatus as shown in the previous figures.
2. Turn on the laser (unpolarized red laser) and work in a dark surrounding.
3. Turn the analyser to be 90° with the polarizer.
4. Increase the angle 10° and after each increase, record the voltage of the light
reaching the detector.
5. Plot a graph between U/Umax as y-axis, and θ as x-axis.
6. Plot another graph between the corrected voltages and cos2 θ.
Data and Calculation:
θ
90
80
70
60
50
40
30
20
10
0
U(mV)
7.3
26.7
50
71.2
108
119
120.8
126
135
138
cos2 θ
0
0.03
0.117
0.25
0.4
0.58
0.75
0.88
0.97
1
0.053
0.19
0.36
0.51
0.78
0.86
0.87
0.91
0.97
1
U/Umax
Table 1
U/Umax
1.2
U (mV)
160
140
1
120
0.8
100
0.6
80
60
0.4
40
0.2
θ
0
20
cos2 θ
0
0
20
40
60
80
100
0
0.2
0.4
0.6
0.8
1
Graph 1
Conclusion:
1. From the graph 1 we can conclude that the closer the angle θ is to 0 the closer
the voltage is to its maximum value.
2. From graph 2 we can conclude that U and cos2 θ are proportional and therefore I
and cos2 θ are proportional which verify Maul's Law.
1.2
Graph 2