Optics Experiment 3
Polarisation of Light
Safety
Make sure that you have read the safety notes in the introductory section of this manual before
beginning any practical work.
Do not, under any circumstances, attempt to repair any of the equipment should you think it to
be faulty. Rather, turn off the apparatus at the power point and consult your demonstrator.
In the course of this experiment you should note that the lamps you use will become very hot. Be
careful to make sure you do not burn yourself.
References
121/2:
Chapter 36 (introduction); Sections 36.2 and 37.4.
141/2:
Sections 34.1, 34.2 and 34.6.
(Do not worry too much about the mathematics in the above references, just try to get a good
grasp of the physical principles).
Introduction
The purpose of this experiment is to:
(i)
give you experience in making and recording observations of phenomena in a logical
fashion (and in drawing conclusions on the basis of these observations); and
(ii)
provide some experience of the phenomenon of polarisation of light.
Physics 121/2 and 141/2 Laboratory Manual
O-31
Background Theory
Light travels as a transverse wave. This means
that the oscillations of the electric fields that
make up light are perpendicular to its
direction of motion, as in figure 1. The magnetic
fields are perpendicular to both the electric field
and the direction of propagation.
y
B
x
E
In this figure each pair of vectors along the z-axis
represents the direction and magnitude of E and
B in the plane intersecting the axis at that point.
A representative plane is shown.
z
The light wave drawn here has all its electric
field vectors lying in one plane, (in this case the
x-z plane), and is hence called plane polarised
light.
Figure 1
direction of propagation
of wave
Looking "head on", an ordinary unpolarised beam of light's electric field
vectors would take up all possible directions in the plane perpendicular to
the direction of motion, and would resemble:
E
whereas plane polarised light's electric field vectors would look like:
or
or
etc.,
depending on the angle of polarisation.
Since the electric field can be represented as a vector, we can also take components of it, ie.

E
Ey
where,
Ex = E cos 

Ex
Ey = E sin 
(Remember that the wave is travelling along the z axis and is
transverse, so Ez = 0.)
The intensity of light that is observed is proportional to the square of the amplitude of the
electric field vector:
I  E2
Section A:
Observations and Hypothesis Formation
In Section A of today’s experiment you will be drawing your own conclusions based on your
observations. To communicate this effectively a clear and logical record of this sequence will be
needed. You should set out your note book as follows (three columns perhaps?).
O-32
Physics 121/2 and 141/2 Laboratory Manual
(i)
Test
Describe the experimental test, preferably using a diagram.
(ii)
Observations
Describe your observations briefly, again using diagrams where
possible.
(iii)
Conclusion
If you can, state any conclusions made on the basis of your
observations. Back up your conclusions with evidence.
Terminology
To help you in writing your account a few items of terminology may be helpful

A polaroid is said to "polarise" the light passing through it.

When “direction of polarisation” is referred to, it conventionally refers to the direction of
the electric vector which has not been "polarised out" of the light; ie. the surviving electric
vector component.

When two polaroids are placed so that minimum light passes through them, they are said
to be "crossed".
Experiment (i)
Uncertainties
There are no measurable experimental uncertainties throughout today’s experiment as the
observations made are qualitative.
Using a light source and the three polaroids provided, devise a series of simple experiments to try
to lead you to an hypothesis, or model, to explain what is happening. For example,
(i)
Experiment with placing one polaroid between a desk lamp and your eye. Describe what
you do and what you observe. In your conclusions section outline what you conclude about
the light emitted by the lamp.
(ii)
Now experiment with two polaroids, recording as above. Include changing the relative
orientations of the two polaroids. Again describe your observations and outline your
hypothesis to explain what you see.
(iii)
Finally experiment with adding a third
polaroid, again recording test,
observations and conclusions.
pair of crossed polaroids
One of the tests you should include at
this stage is the rotation of a polaroid
when placed between two other
polaroids which have first been placed
in the "crossed" position.
Do these observations fit your earlier
model? Does it matter where you place
the third polaroid?
third polaroid
If your model does not stand up to the
Physics 121/2 and 141/2 Laboratory Manual
O-33
test, formulate a new one to fit all your data.
At this point you should discuss your model with your demonstrator. They may
demonstrate the main features of your experiment using microwaves. If so, describe
the demonstration and observations in your practical notebook.
Exercise
O-34
Using the fact that the intensity of light is proportional to the square of the
amplitude of its electric field vector, calculate the intensity of light that passes
through the three polarisers you investigated above. Use two crossed polarisers
with the third polariser sandwiched between them having its polarisation
direction at an angle of 45 degrees to each.
Physics 121/2 and 141/2 Laboratory Manual
Section B:
Polarisation by Scattering
Consider light scattered by small particles and water molecules in a container of water.
beam scattered at
°
90 (polarised)
vertical components
of the electric field
vector for the
incident beam
z
V
H
incident beam
(unpolarised)
V
H
H
o
90
transmitted
beam
K
V
V
o
45
horizontal
components of
the electric
field vector for
the incident
beam
P
H
beam scattered at
90° (polarised)
Q
in the horizontal
plane
The incident beam can be considered as a wave having an associated vibrating electric field
(unpolarised) and, therefore, containing components equally in all directions perpendicular to the
direction of the beam. This is represented in the diagram by resolving the randomly oriented
fields of the incident beam into their total vertical component, V, and their total horizontal
component, H.
Consider the particles that are scattering the light to include electric charges. When the incident
vibrating electric field acts on the charges in the scattering particles, they vibrate in response to
the field. According to electromagnetic theory, these oscillating charges in turn radiate an
electromagnetic wave.
For the case of an incident beam as shown, the horizontal scattered beam to P is polarised
"vertically", while the vertical scattered beam to Z is polarised "horizontally" with components
only perpendicular to the incident beam, and with no component parallel to the incident beam.
The transmitted beam, like the incident beam, is unpolarised.
Experiment (ii)
You may find that a table is useful in recording your observation for the following exercises.

Using a polaroid, observe light scattered perpendicularly from cloudy water, ie. towards Z
and P on the diagram. Compare the polarisation of light directed towards Z and P. In this
way calibrate the polaroid, ie. identify which direction across the face of the polaroid, with
respect to the painted dots, is parallel to the direction of polarisation of the transmitted
light (ie. at right angles to the position of minimum transmission).

Predict what should happen in the direction Q, which is in the horizontal plane, midway
between directions K and P. Record your actual observations.

Polarise the incident beam in the above situation, so that its direction of polarisation is
horizontal. How do you know this? Now make observations at P, Q, Z and K. Describe and
explain your observations.
Physics 121/2 and 141/2 Laboratory Manual
O-35

Repeat for an incident beam with a vertical direction of polarisation.
Question (a)

In which direction would you look in order to be most likely to detect
polarisation in the light from the sky? Draw a diagram to illustrate your
answer.
Using your calibrated polaroid, attempt to detect polarisation in the light from the sky.
Describe and explain what you observe.
Section C:
Polarisation by Reflection
When unpolarised light is reflected from a plane surface it is partially polarised. As is the case in
scattering, the electric and magnetic fields of the incoming beam interact with charged particles
in the surface layer of the material, causing them to oscillate and reradiate. Some of the
re-radiated light is transmitted into the material, ie. refracted, and some is reflected. The degree
of polarisation of the reflected ray depends strongly on the direction of the light within the
material because this direction controls the direction of oscillation of the radiating particles..
Experiment (iii)
normal


A demonstration designed to enable
you to view light reflected at an
angle of 90° from the direction of the
refracted ray is available. The
incident angle for which this occurs
is called the Brewster angle. What
you observe is illustrated in the
diagram. At this angle the reflected
light is completely polarised. Use
your calibrated polaroid to confirm
this.
incident ray
(unpolarised)
reflected ray
(polarised)
P
90°
2
Now use your calibrated polaroid to
observe the light reflected from a
black tile. Then rotate the polaroid.
Vary the angle of the incidence of
the light by varying the position of
your eye. Describe fully what you
observe. Repeat the observations
using a polished metallic surface.
refracted ray
(slightly
polarised)
Summarise your observations and their explanation.
Section D:
Observations with Calcite
Experimental (iv)

Draw a small cross on your notebook, with arms about 3 mm long, and place the crystal
over this cross. Rotate the crystal, keeping the same face in contact with the paper.
Question (b)
O-36
What physical properties must the calcite possess to produce two images from
a single point? Which image appears to be "deepest", and what does that tell
Physics 121/2 and 141/2 Laboratory Manual
you about the refractive indices?
Make note of all you observe, and then discuss the properties of calcite with your demonstrator.
Look at the images through a polaroid - comment further.
Further Work
Observations using crossed polaroids
Your demonstrator will show you the effect of placing certain transparent objects between crossed
polaroids. It is interesting to think about applications of this phenomenon. An understanding of
option D is a necessary basis for understanding the phenomenon. Record your observations and
explanations.
Physics 121/2 and 141/2 Laboratory Manual
O-37