Inverse Square Law

The inverse square law describes how light intensity varies with distance. According to this law, the intensity of light is inversely proportional to the square of the distance from the light source. As the distance between the light source and the plant increases, the light intensity decreases.

Inverse square law can be represented by the equation:

For example, if a lamp is positioned 2 cm from a plant, the light intensity will be:

\left( \dfrac{1}{2}\right) ^{2}=\dfrac{1}{4} or 0.25

In this case, we are implying that the reference distance is 1 cm. This is because when the distance is 1 cm, the intensity is:

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

Essentially, when we increase the distance of the lamp from 1 cm to 2 cm, the light intensity becomes one-fourth or is 0.25 times its original value.

The table below shows the result of increasing the distance of the lamp further:

Distance from Lamp (cm)Light Intensity

The further we move the lamp from the plant, the weaker the light intensity becomes due to the inverse square law.

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 can be determined by dividing the volume of oxygen produced by the time taken to produce it:


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}= \dfrac{15\:cm^3}{5\:min}

= 3 cm³/min

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