Inverse Square Law

The inverse square law represents the relationship between light intensity and distance. The rate of photosynthesis is inversely proportional to the distance from the light source. As the distance between the light source and the plant increases, the light intensity decreases.

This means that light intensity is inversely proportional to the square of the distance and inverse square law is represented by the equation:

So, if a lamp is 2 metres away from a plant, then the light intensity of the lamp is \dfrac{1}{4} of its original value.

\left( \dfrac{1}{2}\right) ^{2}=\dfrac{1}{4}

Measuring the Rate of Photosynthesis

We can calculate the rate of photosynthesis by measuring the volume of oxygen produced by a plant in a given time. The rate of photosynthesis is the volume of oxygen divided by the time taken:


15cm³ of oxygen is collected from a plant in 5 minutes. Calculate the rate of photosynthesis.

The rate of photosynthesis = \dfrac{15\:cm^3}{5\:min}

= 3 cm³/min