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

Additive colour mixing


Mixing coloured light and observing coloured objects in that light.

Apparatus and materials

Small projection lanterns (35 mm slide projectors), 3

Slides for projectors (see technical notes)

Small pieces of colour filter (6 cm x 6 cm) red, green, true blue

Shielded rheostats or variacs for projectors - or alternative (see technical notes), 2

White screen or blank wall

Health & Safety and Technical notes

The rheostats used to control mains electricity must use proper mains plugs and sockets. It is probably easier to use cards with different apertures to restrict light output.

1 The slides are squares of card or metal with a hole about 25 mm diameter. Some method of changing the brightness of one or two of the patches is necessary. A finger or piece of card held in front of the projection lens to reduce its aperture does the job nicely.  

2 Prepare the apparatus before the class assembles. Place the slides in the projectors and focus each to make a round patch of light on the screen. Hang one filter in the front of the projection lens of each. (If the projectors are not equally bright, hang the blue filter on the brightest.) Arrange the projectors so that the three patches of coloured light overlap with a central 'triangle' illuminated by all three. Darken the room completely. Run the 'blue' projector at full voltage. Adjust the other two projectors (with rheostats, variacs or otherwise) until the central triangle looks white. Leave the settings fixed and turn off or cap the projectors. 

Apparatus set-up

 3 Ray box kits are often sold with a version of this experiment. Slots hold red, green and blue filters on the front and sides of the box. The light is reflected by mirrors onto a screen so that the three colours overlap. Tungsten filament lamps have a lot of red light and very little blue light, and so two red filters should be put into the same slot to cut down on the intensity of the red light. The blue filter should be placed in the front slot so that it is as near to the screen as possible. 

Projector with slides

Projector lantern on
Photographs courtesy of Mike Vetterlein


a Turn on the red and blue projectors. Magenta can be seen where the patches of red and blue overlap. Similarly, try blue and green; and finally red and green. 

b Then turn on all three projectors. Wave a hand in front of the white patch, making coloured shadows. 

Red and blue projector
c Turn on the red and blue projectors to make a large patch of magenta. Hold pieces of coloured cloth in that light: first magenta cloth, then red cloth, then green cloth. Ask for explanations. 

Teaching notes

1 Red light + green light = yellow light 
red light + blue light = magenta light 
green light + blue light = cyan light 

red light + blue light + green light = white light 

Complementary colours are two colours which add together to form white, e.g. red and cyan. 
2 A red filter lets through red but subtracts green and blue. Red can therefore be called minus green and minus blue and so minus green and minus blue, is what colour printers call red.  

This experiment was safety-checked in January 2007

Related guidance

Classroom management in semi-darkness

The electromagnetic spectrum

Learning from spectra


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