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Suggested
Teaching Activities:
Polarisation
of Light
1. Introduction
Perhaps
you have seen a display of polarised sunglasses in a store.
You can quickly test to see if the glasses are really polarised
by looking through the lenses of two glasses and rotating one
pair by 90°. If both pairs of glasses are polarised, the
lenses will appear to go black. Why is that?
To
explain the darkened lenses, we need to think of the light as
an electromagnetic wave. An electromagnetic wave has varying
electric and magnetic fields perpendicular to the direction
the wave is travelling. This experiment focuses only on the electric
field variation, represented by a vector. Light emitted from
a typical source such as a flashlight is randomly polarised,
meaning that the electric vector points in varying directions.
An
ideal polarising filter will remove all electric fields
except those that are parallel to the axis of the filter. The light remaining
is then said to be polarised. A second filter can be used to
detect the polarisation; in this case, the second filter is
called an analyser. The transmission through the second filter
depends on the angle between its axis and the axis of the first
filter. In this experiment you will study the relationship between
the light intensity transmitted through two polarising filters
and the angle between the filter axes.
2. Objective
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Observe
the change in light intensity of light passing through crossed
polarising filters. |
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Measure the transmission of light through two polarising
filters as a function of the angle between their axes and
compare it to Malus Law. |
3. Equipment
List
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Datalogger
interface |
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Datalogger
software |
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Light
sensor |
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Light
source |
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Polarising
filters (2) with marked axes |
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Protractor |
Procedure
| 1.
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Place
the light source, polarising filters, and light sensor so
light passes through the filters and then into the sensor.
You will rotate only one filter to change the transmission;
the other filter, the light source, and the sensor, must
not move. Turn on the light source.
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| 2.
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Connect
the light sensor to the datalogger interface. If your sensor
has a range switch, set it to the 600-Lux range.
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| 3. |
Open
the experiment worksheet using the datalogger software.
Light intensity is plotted versus analyser angle.
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| 4. |
Rotate
the analyser until the Light Sensor reading is maximised.
If the reading is larger than 600 Lux, reduce the intensity
of the light source and rotate the analyser again for maximum
sensor reading. This is your zero angle. The axis marks
on the two filters should be parallel.
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| 5. |
Set
the filters so their axes are at right angles. Very little
light should get through the pair of filters. Define the
light level as zero. The intensity reading should now be
near zero.
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| 6. |
Return
the analyser to the parallel position. Start the collection of data
in the datalogger software.
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| 7. |
Rotate
the analyser by 15° record the angle at 15°. Repeat this
process until you have rotated the analyser through one
revolution, or 360° before stopping the data collection.
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Analysis
| 1.
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Place
one polarising filter on top of a second so you have to
look through both of them. Rotate the top filter until the
axis marks are at right angles to one another. What do you
notice? |
| 2.
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Rotate
a filter so the axes are now parallel. Look through the
stack and rotate the top filter about your viewing axis
by 180°. Make a qualitative graph of the transmitted
light intensity you observed as a function of the angle. |
| 3. |
Describe
your graph of light intensity versus. angle, giving important
patterns and points. |
| 4. |
In
the 1800s Malus proposed
to
predict the light transmission through two polarising
filters, where  is
the intensity when the angle 
between the polariser axes is zero.
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| 5. |
Compare
the data to the model. Is your data consistent with Malus'
Law? |
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