Education and Manpower Bureau
Using Datalogger in the Teaching of Physics

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

 Observe the change in light intensity of light passing through crossed polarising filters. 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

 Datalogger interface Datalogger software Light sensor Light source Polarising filters (2) with marked axes Protractor

Procedure

 1. 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. 2. Connect the light sensor to the datalogger interface. If your sensor has a range switch, set it to the 600-Lux range. 3. Open the experiment worksheet using the datalogger software. Light intensity is plotted versus analyser angle. 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. 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. 6. Return the analyser to the parallel position. Start the collection of data in the datalogger software. 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.

Analysis

 1 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 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. 5 Compare the data to the model. Is your data consistent with Malus' Law?

2003