PicoScope 7 Software
Available on Windows, Mac and Linux
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.
DrDAQ datalogger with PC running PicoScope and access to light sources, such as:
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:
Investigate how the light intensity varies with time from the following sources:
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.
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.