Measuring the effects of light colour on photosynthesis is a fun and simple experiment for secondary school or sixth form pupils. By growing the plants beneath colour filters, you can determine which colour of light produces optimal growth.

Visible light is a form of electromagnetic radiation with wavelengths ranging from 400 to 750 nanometres. With a wavelength of about 400 nanometres, violet light is at one end of this region while red light is at the other. Colours like green, yellow and orange lie in between. To select for certain colours of light, you can buy colour filters from science or educational suppliers -- or make your own by using coloured clear plastic.

The first step in photosynthesis is the absorption of light by pigments like chlorophyll A and chlorophyll B. Each pigment absorbs certain colours of light more efficiently than others do. Chlorophyll A and B preferentially absorb blue, violet and red light; carotenoids help to extend the range of colours absorbed by the leaf, but ultimately green light is poorly absorbed. Chlorophyll molecules reflect green light, which is why plant leaves appear green.

Plants grown under blue, violet or red light should out-perform plants grown only under green light. Plants under green light will not be able to use most of the light energy that strikes their leaves, so they will grow poorly or even die.