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Australian
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Education Sub-Committee
Light Polariser
You have heard of Polaroid sunglasses, and the claim that they eliminate the glare of reflected
sunlight. These activities will show you how they work, and introduce some other applications of
polarisers.
A. Making the polarising viewer
For this activity you need:
two polarising filters, one large and one small
polystyrene block, to act as a holder for the polarising filters
sharp knife or blade, ruler and ballpoint pen
felt pen
incandescent globe or other light source
1. Take the polystyrene block and, using a ballpoint pen, mark two lines
across it two centimetres from each end. Place the polystyrene block
on a bench-top with a suitable protective surface. Using a ruler and
the sharp blade provided, carefully make a fine cut along each of the
lines, about 2 mm into the polystyrene block.
BE VERY CAREFUL. Leave the polystyrene block on the benchtop while you are making the cuts, and keep your hands well away
from the cutting blade. DO NOT move the blade towards you as you cut: move the blade
from one side to the other ACROSS your body.
Mark the front (F) and back (B) of the polystyrene block.
2. Select the larger square polarising filter. Use the felt pen to mark a
small arrow in one corner, PARALLEL to one of the sides of the
square. Mount this filter in the back slit of the polystyrene block, so
that the arrow is in one of the top corners.
3. Hold the polystyrene block at the front, and view the light source
through the filter. Rotate the polystyrene holder as you look at the
light source through the filter. Do you notice any changes in the
appearance of the light? Is the change sudden or gradual?
4. Place the second smaller polarising filter into the front slit in the
polystyrene block.
5. View the light source again through both filters. This time, rotate the
front filter in its slit as you look at the light source. Don’t move the
back filter.
6. You should find that there is one orientation of the front filter, which
almost totally prevents light passing through the two filters. This is
referred to as extinction – the light is extinguished. At other orientations some light is
visible.
7. Rotate the front filter to find the orientation at which the MOST light is visible. At this
orientation the two filters are said to be parallel. With the front filter in this parallel
orientation mark an arrow on it, at the same position and in the same direction as the arrow on
the back polarising filter.
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8. When extinction occurs the two filters are said to be crossed. At extinction the arrows on the
two filters should be at right angles. Check to see that this is the case.
9. With the two filters crossed, view the light source while rotating the viewer as a whole. Does
this make any difference to what you see? Try the same with the two filters parallel.
B. Investigating with the polarising viewer
For the next two activities you only need one polarising filter in the viewer. Remove the front
filter, and leave it in a safe place so it won’t get scratched or fall onto the floor.
1. Reflected light
Find a shiny non-metallic surface that is reflecting light from the sun or an inside source.
Suitable surfaces include laminex, polished wood, opaque plastic or a sheet of clear plastic,
perspex or glass over a sheet of black paper.
i. Look at this reflecting surface through the polarising filter. Don’t look straight down – look
at an angle.
If you rotate the filter, you should find that, as the orientation of the filter is changed, the
amount of reflected light that passes through it varies. Find the orientation of the filter for
which the least amount of light is transmitted. Place the filter in the viewer so that you can
hold the filter comfortably at this orientation.
ii. Keep the filter at the orientation you have just found, but change the vertical angle at which
you view the reflected light. Can you find an angle for which almost no light passes through
the polarising filter?
iii. Now try looking at the light reflected from a metal surface. Is this light polarised? Does the
amount of polarisation depend on the angle at which you view the surface, as it did in ii
above?
iv. What about light reflected (scattered) from a non-shiny surface such as matt white paper or
cloth? Is this polarised?
2. Scattered light
Shine a beam of light through a glass container filled with a cloudy solution. Most of the light
will pass directly through the solution, but some of it will be scattered. This scattered light
makes it possible for you to see the beam as it passes through the solution.
i. Look at the scattered light SIDE ON to the beam, through the polarising filter in the
viewer. Rotate the viewer. You should see that the brightness of the beam changes as
you change the orientation of the viewer. Find the angle for which the least amount of
light is transmitted through the filter.
ii. Try viewing the beam from the other side, and from above the container. Is the
orientation of the viewer that passes the least amount of light the same in every case?
What about if you view the beam end on, from the side of the container opposite the
source?
iii. Get one member of your group to hold the second polarising filter between the light
source and the solution, with the arrow pointing either up or down. Again view the
scattered light from each side and above the container, and opposite the source, rotating
the viewer in each position.
iv. Repeat iii, but this time with the arrow pointing sideways on the polarising filter near the
light source.
3. Scattered sunlight
If you can, go outside to look at the sky, or look through a large clear window. Look at an area of
blue sky through the polarising filter in the viewer. Don’t look at the sun itself - stand so that the
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sun is shining from your side. Rotate the viewer to find the orientation that passes the least
amount of light.
4. Optical activity
Shine a beam of light through a clear container of water. View the light emerging from the water
with one of the polarising filters in the viewer. Hold the second polarising filter between the light
source and the container. Rotate the polarising filter in the viewer until the emerging beam of
light is extinguished. When this occurs the arrows on the two filters should be at right angles.
Check that this is the case.
This shows that the water does not affect the polarisation of the light: extinction occurs for the
same orientation of the filters, with or without the water present.
Now try the same thing with a container of concentrated sugar solution. You should now find
that the filters have to be in a different orientation for the emerging light to be extinguished. The
sugar solution changes the polarisation of the light: the stronger the solution, and the longer the
path of the beam of light in the solution, the more the polarisation of the light is changed.
C. Polaroid art
For this activity you need to cut a piece 4–5 cm square from a sheet of overhead projector
transparency.
i. Remove the two the polarising filters from the polystyrene block, and place them on top of
one another so that their directions are crossed. Now place the transparent sheet between the
crossed filters to make a sandwich.
Look at the light source through the sandwich. Rotate the transparent sheet, while keeping
the polarising filters crossed.
Many brands of transparent sticky tape also produce the same effect.
ii. Take a microscope slide, and place two short lengths of sticky tape on it so that they overlap
partly, but not completely. Place the microscope slide between the crossed polarising filters.
Look at the light source through the sandwich and rotate the microscope slide.
You can produce some artistic stained glass effects using patterned layers of sticky tape – try
it!
iii. Reassemble the polarisers in the viewer, with their directions crossed. Hold a clear plastic
object (such as a ruler, or setsquare, a CD cover etc) between the polarisers. You will see
some particularly interesting effects around any cracks or holes in the plastic. Rotate the
plastic object between the crossed polarisers.
iv. Try holding some plastic wrap or cellophane between the crossed polarisers, and rotating it.
What difference does it make when you stretch the plastic wrap? Fold some of the plastic
wrap in layers, and view it between the crossed polarisers. Try scrunched up plastic wrap or
cellophane.
v. Place a piece of waxed paper or other translucent material between the crossed polarisers.
The translucent material scatters and depolarises the light. How can you tell that the effect
you see is due to depolarisation of the light, not birefringence?
This activity was designed by David Mills, Keith Thompson (both of Monash University, Department of
Physics), and Dan O’Keeffe (Camberwell Grammar School), and written by Christina Hart, for the
Education sub-Committee of the Australian Institute of Physics (Victorian Branch). Layout and graphics
by Bronwyn Halls.
 The Australian Institute of Physics (Victorian Branch), 1999.
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Switched
On
To
Physics
Australian
Institute of
Physics
Education Sub-Committee
Light Polariser
Explaining polarisation
Electromagnetic waves
You probably already know that light is said to be a form of wave energy, but to understand
polarisation you need to know a little bit about the kind of wave that physicists have in mind
when they say this.
As with any form of energy, light can be produced in one place and have an effect in another
place, which may be very distant from where it was produced. In this respect it is similar to a
tennis ball that carries energy from the place where it is hit to the place where it lands. A sound
wave also carries energy from the place where it is produced, (such as your throat) to the place
where it is heard (your friend’s ear). But whereas the tennis ball moves with the energy it carries,
the air particles do not travel with the sound wave as it moves. The air particles jiggle around as
the sound energy moves past them, but the sound is the disturbance that passes through the air,
not movement of the air itself as in a gust of wind. In the same way, a ripple momentarily
disturbs the water’s surface, but the water does not move along with the ripple. One of the big
questions in physics, since the time of Isaac Newton (1642-1727), is whether light is made up of a
stream of particles that (like the tennis ball) carry energy with them as they move, or whether
(like sound and water ripples) light travels as a disturbance or wave.
There were several reasons why, by about 1850, physicists eventually decided that light must be a
form of wave energy1. However, unlike sound, light can travel through a vacuum, and this makes
it a bit difficult to see what is ‘waving’, or being disturbed, as the light travels. In fact, this was
one reason why Newton himself thought light must consist of a stream of very tiny, very fast
moving, particles. James Clerk Maxwell, whose eminence in physics is comparable with that of
Newton and Albert Einstein, found an answer to this question in 1864. In doing so, he also
showed mathematically that light must belong to a much larger family of waves, called
electromagnetic waves, that are produced by vibrating electrons. Other members of the family
include radio waves, microwaves, ultraviolet-waves and x-rays.
The theory that Maxwell proposed is rather a difficult one to explain fully; it is usually studied in
university physics courses. But it depends on the idea that all charged particles, including
electrons, have an electric force field associated with them. Any other charged particle nearby
experiences an electric force from the force field. (In some ways the electric force field is like the
gravitational force field that exists all around the earth. But while anything in the earth’s
gravitational force field will feel a gravitational force from the earth, only charged particles can
feel one another’s force fields.)
When an electron vibrates, it produces a
disturbance in the electric force field - in other
words a wave. (It also produces a magnetic
field disturbance, which is not important here,
but which explains why the waves are called
1
Interestingly, by the first decade of the 20th century, several important experiments had shown
that sometimes light did not behave like a wave, but was more like a particle. The Quantum
Theory was developed to account for these apparently conflicting behaviours of light.
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electromagnetic waves.) Figure 1 represents the electric field disturbance, or electromagnetic
wave, that is produced by an electron vibrating up and down along the vertical axis.
All electromagnetic waves travel at the same speed as light waves, but some have a much longer
wavelength. The wavelength of a wave is the distance between crests of the wave (see figure 2).
Radio waves have a much longer wavelength than light waves; x-rays have a much shorter
wavelength than light waves.
wavelength
Figure 1: The varying electric force field produced
long wavelength wave
by a vibrating electron.
short wavelength wave
Figure 2: Long and short wavelength waves
Radio waves are produced by electrons vibrating up and down inside a radio transmitter. The
electromagnetic radio wave travels away from the transmitter and can be picked up by a radio
aerial, as illustrated in figure 3. When the wave arrives at the aerial, it makes the electrons in the
aerial vibrate, and the rest of the radio receiver translates this electrical signal back into a sound
wave.
Electrons vibrating
up and down inside a
radio transmitter
produce an electromagnetic wave, which
travels outwards in all directions from the
transmitter
Some of these waves may
reach a radio-receiving aerial,
in which case the electric
force field disturbance will
make electrons in the aerial
vibrate up and down
radio
transmitter
radio aerial
(receiver)
Figure 3: Radio waves produced by a transmitter, and picked up by an
aerial
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The electric field disturbance is in the
same direction as the motion of the
electrons up and down the transmitter
(and the receiving aerial); in other words
the electric field disturbance is
perpendicular (at right angles) to the
direction in which the wave is travelling,
as shown in figure 4. The same is true
for all other forms of electromagnetic
waves, including light waves: the electric
field disturbance is in the same direction
as the vibration of the electrons, and is at
right angles to the direction in which the
wave travels.
Direction in which
the radio wave is
travelling
Direction of
the electric
force field
Figure 4: The direction of the electric field variation
is perpendicular to the direction in which the wave is
travelling
Electrons in the surface of
the filament may each be
vibrating in different
directions
In an incandescent light globe the
electromagnetic wave (light) is produced
by vibrating electrons near the surface of
the tungsten filament. The vibrations of
these electrons are very small compared
to the vibrations in the radio transmitter,
and different electrons will each be
vibrating in different directions. Each
electron produces a bit of light wave with
an electric field disturbance in the same
The electric field
direction as that electron’s own vibration.
disturbance can be in all
Overall, as shown in figure 5, the electric
directions that are
field vibrations in the light from the
perpendicular to the
filament can be in any and every
direction in which the light
direction that is perpendicular (crosswise)
is travelling
to the direction in which the wave is
Direction in which
moving. Electromagnetic waves like
the light travels to
these are said to be unpolarised.
the observer
Electromagnetic waves like those from
the radio transmitter, where the electric
field disturbance is in one direction only,
are said to be polarised.
Figure 5: Light from an incandescent globe is
When light passes through a polarising
unpolarised
filter, the filter only lets through the light
waves where the electric field vibration is
in one particular direction. So the unpolarised light coming from the light source is polarised by
the filter. If two polarising filters are used, and their polarising directions are the same then the
polarised light from the first filter can also pass through the second filter. If however the
polarising directions are at right angles, as in figure 6, then the polarised light that comes through
the first filter cannot pass through the second.
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The first polarising
filter only lets
through electric
fields in this
direction: it
polarises the light in
this direction
The second polarising
filter can only pass
light polarised in this
direction, so no light
gets to the observer
Figure 6: Crossed polarisers in front of a light source
Explaining those Polaroid sunglasses
When light falls onto a shiny, opaque, surface some
of the light is reflected, and some is absorbed by the
reflecting surface. The incoming light ray is
unpolarised. So the electric field disturbance is in
all directions perpendicular to the direction of the
incoming ray. Figure 7 shows only the vertical and
horizontal directions, but this is enough to show
what happens.
The light energy is absorbed by electrons in the
atoms of the reflecting material. So these electrons
start to vibrate and send out light energy. It turns
out that the electrons which are set vibrating by the
horizontal part of the electric field disturbance are
slightly more likely to send out the light that we see
Figure 7: Light reflected from a non-metallic
as reflected. On the other hand, the electrons that
surface is partially polarised
are set vibrating by the vertical part of the electric
field disturbance are a lot more likely to produce energy that will be absorbed by the reflecting
surface. This means that nearly all of the vertically polarised light is absorbed by the reflecting
surface, leaving the reflected light somewhat horizontally polarised. For one particular angle of
reflection the reflected light is completely horizontally polarised – all of the vertically polarised
light is absorbed by the surface. This angle is known as the Brewster angle, after the person who
first found it experimentally.
Polaroid sunglasses have lenses made from polarising filters, positioned so that horizontally
polarised light will not pass through. This means that you do not see most of the sunlight that is
reflected directly from the sand or water and most other horizontal surfaces. All sunglasses
absorb some of the light coming to your eyes, but only Polaroid sunglasses are able to reduce the
glare from reflected light more than they reduce the amount of light that reaches your eyes in
other ways.
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When unpolarised light is shone into the cloudy solution, the particles in the solution absorb the
energy from the electromagnetic wave. This energy makes the electrons in the atoms of the
solution vibrate in the direction of the electric field force, and the vibrating electrons then send
out light waves. Remember that the electric field disturbance must be perpendicular to the
direction in which the light is travelling. So, the light shining on the solution can only make the
electrons vibrate up and down, or from side to side, or anything in between, but not backwards
and forwards in the direction of the light beam itself. The electrons that vibrate up and down will
send out vertically polarised light sideways, while the electrons that vibrate from side to side will
send out horizontally polarised light directly upwards.
If the light shining into the solution is vertically polarised, then scattered light can only be seen
Figure 8: Unpolarised light shining into a cloudy solution, compared with light that is vertically
polarised
from the side, not from above, as shown in figure 8. Horizontally polarised light can only be seen
from above.
The air molecules that make up the earth’s atmosphere scatter light from the sun in much the
same way as the particles in the cloudy solution. So the light that comes from the sky is at least
partly polarised. Human eyes are not sensitive to the polarisation of light, so you do not see that
the light from the sky is polarised, unless you look at it through a polarising filter. However,
photographers sometimes use a polarising filter on their camera, which prevents the polarised
light from the sky reaching the film. This makes the sky appear just slightly darker than it would
without the filter, and enhances the contrast with the bright clouds.
When you look at the light scattered from the cloudy solution, the degree of polarisation, and the
direction of the polarisation depend on where you look from. The same applies in the
atmosphere: the amount and direction of the polarisation in the light from the sky depends on the
direction in which you are looking relative to the sun. A solar compass can find the direction of
the sun by analysing the polarisation of the light coming from the sky. This provides a means of
determining direction, and is especially useful near the earth’s poles where a magnetic compass is
useless.
Many insects seem to navigate by the polarisation of light from the sky. The wiggle dance of
bees, in which they let others in the hive know where food can be found, seems to depend on their
being able to ‘see’ the polarisation of light. In fact, bees may see patterns of polarisation as a
pattern of colours.
Birefringence and optical activity
Some substances - like the sticky tape and transparency sheet - change the direction of
polarisation of light. This allows some light to pass through the crossed polarising filters. The
direction of polarisation is changed by a different amount for different colours, which is why you
see coloured effects. The thickness of the material also affects the amount by which the direction
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of polarisation is changed, which is why overlapping layers of sticky tape give rise to different
colours.
This effect is a rather complicated result of an unusual property of materials like the transparency
sheet. Light travels at different speeds through these materials, depending on its direction of
polarisation, and they are referred to as birefringent materials.
Many biological substances, such as proteins and nucleic acids, cannot be seen with a normal
microscope. However, they are birefringent, and so they can be seen in a polarising microscope,
which has built in polarising filters.
Some substances are not normally birefringent, but can be made so by stretching, twisting or
otherwise stressing them. This provides a useful way of investigating the forces in machine parts.
A plastic model of the part is made, and viewed between crossed polarisers. Then the same kinds
of forces are put on the model that the real part will get in actual use. The rainbow pattern that is
seen can be used to work out where the forces are most concentrated, and therefore where the part
needs to be made strongest. Other structures such as buildings and bridges can also be tested in a
similar way.
Some other substances are able to rotate the direction of polarised light. Such substances are said
to be optically active. Although optical activity produces effects that are rather like
birefringence, the two things are actually quite different. Molecules in optically active substances
usually have a spiral shape, and the way the spiral is wound affects which way the molecules
rotate the direction of polarisation. Most sugar molecules found in living things rotate the
direction of polarisation to the right, while the proteins found in living things rotate it to the left.
Some more activities with your polariser
Polarisation of direct sunlight
If you were not able to look at light coming from the sky during activity B3 above, try some other
time. This activity shows that light scattered from the sky is polarised.
If you want to investigate the polarisation of direct sunlight it is really important that you DO
NOT LOOK DIRECTLY AT THE SUN. If you do look directly at the sun you may damage the
retina of your eye. This will cause blindness, although the damage may not be immediately
obvious.
A safe way of investigating direct sunlight is to use a pinhole. To do this you will need a piece of
cardboard (from the back of an A4 notepad for example), a large piece of white paper such as
butchers’ paper, a sharp implement such as a meat skewer, and some sticky tape.
Make a small hole in the centre of the piece of cardboard, a few millimetres in diameter. Use the
sharp implement, but keep your hands well clear of the hole as you push the implement through.
Stick the piece of white paper on the ground, in direct sunlight. Alternatively stick the paper on a
wall that is in the direct sunlight.
Stand with your back to the sun so that the piece of paper is slightly in front of you, and just to
one side. Hold the piece of cardboard on the same side as the piece of paper, so that the sunlight
can shine through the hole onto the piece of paper. If the image of the sun on the paper is too
faint, make the hole in the cardboard slightly bigger.
You can investigate the polarisation of the sunlight by getting a friend to hold one of the
polarising filters between the holes and the piece of paper. Rotate the filter to see whether or not
the light is polarised.
(This method of looking at the sun is also safe to use during a solar eclipse. Even if the sun is
obscured during an eclipse, is still dangerous to look directly at the light. You can also observe
sunspots in this way.)
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Polarisation and birefringence outside the laboratory
Use your polarising viewer to investigate the polarisation of light reflected from roads and
pavements, water surfaces, car windows and car bodies, steel, chromium or enamel cookware,
knife blades, aluminium foil, and anything else that interests you that you can view with safety.
If you have an opportunity try looking at a rainbow with your polariser. You should find that the
light reflected from the rainbow is polarised parallel to the rainbow arc. You can make your own
rainbow with the garden hose: just remember that to see the rainbow you have to look at the water
drops with the sun is coming from behind you.
Look around for birefringence effects. Try car windows, glass kitchen ware, a chip of ice, clear
jelly, plastic items such as CD covers and chocolate boxes and any other items mentioned in the
Polaroid art activities that you were not able to investigate then.
You can also look for optical activity in other kitchen substances. Try a salt solution, and
turpentine (but remember turpentine is highly flammable!).
If your school has suitable photographic equipment you can try taking photographs of clouds with
and without a polarising filter on the camera lens. You need a sunny day when there are bright
white clouds in a blue sky. The orientation of the filter on the lens will be important! Water
reflections are a bit harder to come by, but a polarising filter can make a difference to
photographs of such reflections.
More applications of polarisation
To find out more about the uses of polarised light have a look at the home page of the Polaroid
Corporation, at www.polaroid.com. Most of the products now made by Polaroid have little to do
with polarisation of light, but the Company name derives from the polarising material which was
one of its first products.
On the home page select “Polaroid Products”, scroll down to “OEM Products”, and then select
“Applications”. The first three listed applications depend on circular polarisation of light. All
of the activities in the SOS Light Polariser Session were examples of linear polarisation of light.
Circular polarisation is a good deal more complicated than linear polarisation, so the last two
applications will be easier to understand than the first three. However, you can get a good idea of
these other interesting applications without necessarily understanding the details of circular
polarisation.
These notes were prepared by Christina Hart, with help from David Mills, Keith Thompson (both of
Monash University, Department of Physics), and Dan O’Keeffe (Camberwell Grammar School), for the
Education sub-Committee of the Australian Institute of Physics (Victorian Branch). Layout and graphics
by Bronwyn Halls.
 The Australian Institute of Physics (Victorian Branch), 1999.
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