Light & the Rate of Photosynthesis

The intensity of the light available to a plant determines the amount of energy that it has to carry out photosynthesis
- This means that more light a plant receives, the faster the rate of photosynthesis
- In other words, the rate of photosynthesis is directly
  proportional to light intensity
However, the intensity of the light available to a plant depends on how far the light source is from the plant
- The further away a plant is from a light source, the lower the rate of photosynthesis
- In other words, the rate of photosynthesis is inversely
  proportional to the distance from the light source

A light meter can be used to measure light intensity
Light intensity outside (natural light) changes gradually throughout the day as the sun rises and sets
Artificial light sources (e.g. bulbs) can also vary in light intensity when they are at different distances from a plant
As a light source gets further away from a plant (i.e. as the distance from a light source increases), the light intensity decreases
- Also, therefore, as the distance decreases, the light intensity increases
This means that light intensity is inversely proportional to the distance between the plant and the light source
Light intensity actually decreases in proportion to the square of the distance – this is known as the inverse square law
- Therefore, the inverse square law is as follows:

light intensity ∝ 1 ÷ distance²

For example, if the distance between a plant and a light source was doubled, the light intensity would be four times smaller

Here are a couple of tips to help you understand the inverse square law:

The symbol ‘∝’ means “is proportional to”
If ‘x’ is inversely proportional to ‘y’, it is also true to say that ‘x’ is proportional to ‘1 ÷ y’