P&W : Inverse Square Law

Question 1

Irradiance

Answer 1

The power per unit area incident on a surface

Question 2

Experiment ot verify inversee square law for a point source of light.

Answer 2

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 conected to computer and a ruler.

1. Measure the ambient light level with the lamp off. You will need to subtract this reading from all light measuremnts so that your results give light due to the lamp only.
2. Using the ruler as reference, vary the distance, d, of the light sensor from the centre of the lamp.
3. As the area of the light sensor is constant the light sensor reading can be taken as the irradiance I.
4. Plot irradiance against 1/d squared

Question 3

Inverse Square Law

Answer 3

Irradiance is directly proprtional to the inverse of the distance squared for a point source of light.