Inverse Square Law.docx

Light intensity Investigating light intensity

Light provides the energy needed for photosynthesis. Increasing the light intensity increases the rate of photosynthesis, provided plenty of carbon dioxide and water are available. The maximum rate is determined by the other factors involved.

Graph showing rate of photosynthesis and light intensity

The role of light in photosynthesis can be investigated in several ways. In this experiment, a light is moved further from a photosynthesising plant to vary the light intensity the plant receives. The further the light moves from the plant, the fewer oxygen bubbles are produced by photosynthesis.

A light source positioned at a distance from a photosynthesising plant

The intensity of light at different distances from a light source can be described by the inverse square law. This states that the intensity of light is inversely proportional to the square of the distance from the source. In practical terms, this means that when the light is moved twice as far from the plant it will receive a quarter of the energy. Calculating light intensity

Light intensity can be calculated using this formula.

The symbol ∝ means ‘is proportional to’, and distance is measured in metres. Therefore, when the light is 20 cm from the plant, it will receive = 25 arbitrary units But when the light is 40 cm from the plant, it will only receive = 6.25 arbitrary units The SI unit for light intensity is lux. However, different light sources provide different initial values of lux, so the term ‘arbitrary units’ has been used in this example.