Experiment to verify inverse square law for a point source of light
Watch this video to see the correct procedure for carrying out this experiment.
Set up a clear lamp with a small filament as the point source over a black surface in a darkened room with a light sensor and ruler as shown:
- Measure the The light that is already present before the addition of any other light source. level with the lamp off. You will need to subtract this reading from all light measurements so that your results give light due to the lamp only.
- Using the ruler as a reference, vary the distance, \(d\), of the light sensor from the centre of the lamp.
- As the area of the light sensor is constant the light sensor reading can be taken as the irradiance \(I\).
- Plot irradiance against\(1/d^2\)
This graph illustrates the relationship between irradiance from a point source and distance from the source.
The straight line gradient shows that irradiance increases as \(1/d^2\) increases.
That indicates that irradiance is inversely proportional to distance from the point source.