Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Mixing coloured light


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


Health & Safety and Technical notes

Do not look directly into the projector beam.

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

1 Depending on the age and ability of students, this demonstration can take 30-50 minutes. 

2 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. 
3 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. 
4 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. 
5 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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