Learning Aims:
  • Understanding that white light can be produced by mixing red, green and blue light
Overhead projector, sheet of card ~300 x 300 mm with three identical holes approximately 15 x 30 mm in dimension, red, green and blue filters, 3 small plane mirrors, Neutral Density filters with low optical density, magnifying glass
Suggestions for use:

Tape the red, green and blue filters over each of the holes in the sheet of card and position this on the overhead projector to produce three distinct beams of coloured light. Invite students to intercept each primary colour with a mirror, deflect it onto the ceiling or whiteboard and hence observe and note the colour that results when any two beams are mixed and when all three are mixed.

Next, the students should be asked to consider what would happen if the red, green, or blue beam was not as intense as the others – for example, if red was weaker than green, what colour would be produced by mixing them? The students can then test their ideas by placing the neutral density filters on top of each of the coloured filters and mixing the light.

Finally, the students should discuss whether any devices they know of produce different colours by mixing just red, green, and blue light of different intensities. They can verify that a TV, laptop or mobile phone screen does exactly this by examining the pixels with a magnifying glass.

Additional experiment: Using a single red, green, or blue filter and an OHP, project a small coloured spot onto a screen and have the students stare at it for at least 1 minute. Once the filter is removed (and the OHP left on), the students will see a small spot that persists for a moment in their vision that is a different colour to the spot that was projected – most people see red where it was green and vice versa.

The human retina contains cone cells that are sensitive to red, green and blue primary colours. Staring at a red spot breaks down the pigment in the red-sensitive cone cells and when the filter is removed these ‘bleached’ cells will be less sensitive than the green- and blue-sensitive cones, subsequently leading to the persistence of a spot of different colour. This can be used as a demonstration that the eye is sensitive to primary colours and that our perception of colour is due to red, green, and blue mixing.

Possible questions:
  • Is it possible to create white light without using the seven colours of the rainbow?
  • If you can create all visible colours by mixing red, green, and blue, then can you detect all colours by just measuring how much red, green, and blue arrives at a sensor?
  • Is this how the eye sees colour?