Learning Aims:
  • Understanding that white light is composed of many different colours
  • Understanding that filters only allow some colours through
Torches with a narrow cardboard slit attached, glass prisms, good quality (ie theatre quality) red, green, or blue transmission filters, coloured cardboard ‘screens’
Suggestions for use:

Hand out Worksheet 1.4.

Each student will project light from the narrow cardboard slit on their torch through a prism and onto a white sheet of paper. They will see the familiar red, orange, yellow, green, blue, indigo and violet rainbow colours. The students are then presented with two alternative explanations for this phenomena: either the prism ‘converts’ white light into coloured light or white light is a mixture of colours that are subsequently ‘separated’ by the prism through different angles.

Although students may already know the correct interpretation, they cannot distinguish between these alternatives on the basis of their observations. They should then be asked to suggest an experiment that could resolve this problem. One experiment often suggested is the use of two prisms to show that the spectrum of colours can be recombined to give back white light. This should be done as a demonstration although it does not resolve the problem as presented – even with two prisms it is still not clear whether the spectrum of colours is present in the white light before the interaction with the prisms.

The students should then investigate the use of coloured filters. By placing a red, green, or blue transmission filter between the prism and screen they will observe that only the corresponding colour is transmitted. If they now place this filter between the torch and prism, they will observe that red light is transmitted through the filter, passes through the prism, and arrives at the screen. The students can repeat this process with different filters, hence proving that white light is a mixture of different colours and these are spatially dispersed by the prism.

The key to this experiment is the quality of the filters. If this poses a problem in terms of quantity, then the activity could be run as an interactive demonstration with students invited to place filters in the appropriate positions and record the results.

Possible questions:
  • If the sun produces white light, then what must happen to the light to make leaves appear green in summer?
  • In autumn, why do leaves then appear red and orange?
  • How are rainbows formed?