Light, Sound and Waves

Mixing coloured light

Practical Activity for 14-16 PRACTICAL PHYISCS


A way of demonstrating colour addition that does not require filters or lamps.

Apparatus and Materials

  • Data projector and computer
  • Small mirrors, 2 or 3
  • Prism

Health & Safety and Technical Notes

Read our standard health & safety guidance

Works best in a darkened room.


This is a participative demonstration based on the PowerPoint presentation...

Primary Colours

...a resource that is freely available from the TES Connect website. The data projector itself acts as a high intensity light source.

Teaching Notes

  • Depending on the age and ability of students, this demonstration can take 30-50 minutes.
  • When you get to slide 4, split up a beam of white light using a prism. The spectrum looks awesome if projected onto a distant wall.
  • With slide 16, use small mirrors to reflect and combine pairs of the primary colours on a white surface. This takes some practice but it works surprisingly well. It is also possible to overlap all 3 reflected colours to show white, but this is more difficult.
  • With slide 18, ask 'If white light can be split into the full spectrum, why do we only need to combine red, blue and green light to make white?' This leads to a discussion of red, blue and green photo-sensitive cells in the eyes; the after image illusion demonstrates the effects of this.
  • At slide 24 it is worth allowing students time to write how they would design the illusions in the list, to test whether they understand the principle. Once they have decided what colours would go where,- show the working illusions.
  • Slides 21-36: Students love these illusions. Leave the colour-inverted images up for 30 seconds and ask students to stare at them. After the 30 seconds show the following white screen and ask students to blink. They should see an after image with the correct (complementary) colours.

This experiment, based on a presentation available from the website TES Connect, was submitted by Peter Tryon, Tien Shan International School.

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