Flickering lights

Introduction

In this experiment the datalogger is used to sense the rapid change in light levels from different light sources. The frequency of the variation of light intensity can be measured.

This experiment is suitable for pupils following an Advanced Physics course (17–18 years old) as well as showing younger pupils (aged 14+) how electronic sensors can detect changes in light levels that are too rapid for our eyes to detect.

Equipment required

DrDAQ datalogger with PC running PicoScope and access to light sources, such as:

• A filament lamp
• Fluorescent tube
• TV screen
• Computer monitor

Experiment setup

The light sensor on the DrDAQ is placed so that light from the source strikes the light-dependent resistor on the board.

There are two ways in which the frequency of the signals can be measured:

1. Reading from the PicoScope graph display
2. Using the frequency meter

1. Connect the DrDAQ to the parallel port of the PC and run PicoScope. Select Light for channel A (10 ms/div x1). Placing a filament lamp near the DrDAQ should enable a trace to be obtained that shows a periodic nature. To increase the vertical scale change OFF to x 5 and use the slider to locate the trace. To freeze the display click on STOP. Measure the period (T) of the wave and calculate the frequency (f) using f = 1/T (take care with units: if the period is in ms then the frequency will be in kHz).
2. Connect the DrDAQ to the parallel port of the PC and run PicoScope. Select the Meter option and Light and Frequency. Placing a lamp near the DrDAQ light sensor causes the frequency meter to display the frequency at which the intensity of the light is varying.

Carrying out the experiment

Investigate how the light intensity varies with time from the following sources:

1. A filament lamp
2. Fluorescent tube
3. TV screen
4. Computer monitor

Questions and discussion of results

Mains electricity in the UK has a frequency of 50 Hz. So why is the observed frequency for light intensity from a lamp different than this?

The brightness of the lamp is related to the power supplied to the bulb.

The sketch below shows how the voltage (potential difference) for a mains AC lamp varies with time.

Add an axis that shows when V = 0.

The electric current (I) flowing through the lamp is given by V = I x R, where R is the resistance of the lamp.

Add a second curve that shows how I varies with time (do not worry about the vertical scale).

Now, the power supplied to the lamp is given by: Power = current x voltage

Use this equation to sketch a graph that shows how the power varies with time.

Further study

An improved sensor can be made using a photodiode or phototransistor as a light detector. These have a much faster switching time (of the order of 1 microsecond) and therefore can respond more rapidly than the LDR to changes in light intensity.