Datalogging in Science

Polaroid Sunglasses

Overview

The experiment involves measuring the change in light when two polarising filters are rotated relative to each other.

Background

Many sunglasses use Polaroid material, which contains very long chain molecules that act as a Venetian blind and only let light through at a particular angle. These are very effective in removing sparkle and glare. Most light is chaotic in character and when it enters the eye, the human has difficulty working out what it is seeing. The resulting glare or sparkling causes a strain on the eye. A polariser cuts the chaotic light by only allowing light of a narrow band of orientations to pass through. A venetion blind is fairly good analogy. Light after it has been through a polariser is much closer to how the eye likes it.

One disadvantage of polarisers is that they reduce the light to only one plane. If a second polariser is used at right angles to the first no light gets through! This is useful in some applications e.g. LCD units, but in other cases not. You may have noticed if you wear polarising sunglasses that the screens of digital watches, computers, MP3 players or controls on car dashboards become invisible. In most cases this is inconvenient, in the case of an instrument panel it is dangerous.

Apparatus


1. Datalogger.
2. Light sensor (1,000 lux).
3. Low power lamp and power supply.
4. 2 pieces of polarising filter.
5. Protractor (for measuring degrees).

Procedure

Note: You will need to create your own method of supporting the filters and moving one of them. The important thing is to be able to measure the angle you rotate the moveable filter.

1. Set up the apparatus to have one fixed filter and one filter that can be moved (rotated) relative to the fixed one. The light should remain a fixed distance from the filters.


2. Look through both filters at the light source.
Rotate the moveable filter until the light seems brightest.
Place a marker / pointer on both filters to identify this position.

3. Launch the graph software and set it to record in Snapshot mode.


4. Check the two filters are in the position to give maximum light and begin recording.
Check the display and make sure the reading from the light sensor is below the maximum of the sensor range. If the light level indicated is very close to the maximum or showing the maximum value,move the light source away from the filters to give a reading of about 90% of the full scale.


5. Record the angle at which each light reading is taken, as well as the light level in Lux.


6. Rotate the moveable filter by 5 degrees each time.

7. Repeat until the rotating filter has moved 180 degrees.

8. Stop the recording and save the results before analysis.

Results

Plot a graph of light (in Lux) against angle (of filter).
The graph should show a sine wave with a point of high light and point of low / no light.


Questions

1. How does a polarising filter work?
2. Why would using polarising filters help an angler, a photographer and a golfer?
3. Why does the display on an MP3 player of flat screen LCD screen go blank if you wear polarizing sunglasses?
4. Why might it be dangerous to wear a set of polarising sunglasses in a modern car?

Extension

Rotate the polarising filter in front of an LCD and see when the display becomes invisible.

Data Harvest Users
1. Connect the sensor to the datalogger and the logger to the computer
2. Then click the Set Up icon to the right of this message.
3. When the software opens, click the Play button.