The Electromagnetic Spectrum
Light, Sound and Waves

Understanding colours

Practical Activity for 14-16 PRACTICAL PHYISCS


Using filters to produce coloured lights and observe coloured objects.

Apparatus and Materials

For each student or group of students

  • Pieces of each of the following Lee plastic colour filters:
  • Primary red (No- 106)
  • Primary green (No- 139)
  • Primary blue (No- 79)
  • Magenta (purple) (No- 113)
  • Yellow (No- 101)
  • Samples of brightly coloured cloth
  • And...
  • Light source, compact
  • Power supply for light source, low voltage, variable
  • Lens, +6D
  • 60-degree prism, large (preferably high dispersion)
  • White screen or blank wall
  • Translucent screen
  • Low power lamp to go behind screen
  • Small projector to provide large patch of coloured light
  • Set of colour filters (60 mm x 60 mm) for projector
  • Bunsen burner with salt

Health & Safety and Technical Notes

If the demonstration spectrum uses the compact light source, check that the class cannot observe the white light directly.

Read our standard health & safety guidance

The Lee filter numbering system is that used most widely in the stage lighting and photographic business. Sheets of plastic filter about 1 m x 0.5 m can be obtained from photographic and lighting suppliers for less than UKP 5 per sheet (September 2006). One such supplier is: Back Stage Shop.

Pieces of filter about 100 mm x 100 mm are suitable as it is easier for students to look with both eyes: it also lessens confusion in handling if large pieces are used.

Beforehand, use a felt-tipped pen to label each filter R, G, B, C, M or Y.

It will save time and trouble if each set of six different filters is put in an envelope ready to be issued. A key list can be printed on the envelopes: e.g.

R is RED

M is MAGENTA (RED + BLUE) etc.


Examining colour filters

  1. Set up a...

    Demonstration spectrum

    ...Arrange it so that the screen faces the class normally, but that the spectrum falls on the screen very obliquely so that a long spectrum is seen. Position the translucent screen far away from the spectrum screen and place a low wattage lamp behind it as a background to view the colour of a filter. For parts 3, 4 and 5, the rest of the room should be as dark as possible.
  2. With the lab well-lit, examine the filters, looking through each in turn at the lighted translucent screen or the bright sky. Use one filter at a time first, but allow students to experiment with combinations if they wish. Observe a piece of red cloth through a red filter and then a green filter. Repeat this using a green filter.
  3. Primary filters and spectra: Darken the room as much as possible but keep a small lamp running behind the translucent screen so that students can select the filter they want. Look at the spectrum through a red filter, then green, then blue.
  4. Secondary filters and spectra: Repeat 3 using cyan, magenta and then a yellow filter.
  5. With a small projector throw a large patch of red light on the wall by hanging a red filter on the front of the projection lens. Show pieces of coloured cloth in that red light: with no other light in the room. Change to green light (the green filter will transmit a little red as well as all green).
  6. Return to full daylight. Look at the bright screen or sky through pairs of filters in series.
  7. RED and GREEN
  11. Take three large pieces of filter, magenta, cyan and yellow, and tape them on a window pane, overlapping, so that they can act as a reminder to the class.

Teaching Notes

  • Some primary schools study colour so some students may remember some of this.
  • In part 2 you might ask students how the filters make light coloured. Does the red dye change all parts of the spectrum to red, or does it just cut out other colours and leave the red that was always there in the white light? Is the dye a colour adder or a colour subtractor? Filters transmit their own colour and absorb the rest. They do not dye all the light with their own colour. They subtract colour.
  • The dye in a piece of coloured cloth is a selective filter. A piece of red cloth can only return red light to our eyes and look red if it receives some red light. If red cloth is observed in white light or magenta light, its red colour can be seen because each of those contains red light. But if it is observed in just green light, the cloth looks black because it receives no red light to return.
  • The red filter is plain red.
  • The green filter is plain green and is the colour our eyes are most sensitive to.
  • The blue filter is plain blue.
  • The cyan filter is made from blue and green.
  • The magenta filter is made from red and blue.
  • The yellow filter is made from red and green and not the pure yellow of a sodium lamp.
  • In part 3 ask students what each filter does to the various coloured lights. Does a red filter blot out (absorb) parts of the spectrum or does it paint the spectrum red? Does it manufacture more red light or just leave red light that was already there? (It filters so it subtracts.)
  • Using the secondary filters in part 4 to view a white light spectrum, first of all singly, then through two or more filters, will show that secondary filters transmit two colours. Two overlapping filters will transmit only the colour common to both of them. The yellow filter will transmit red and green, as well as a little true yellow. Almost all the yellows are mainly red + green, a subjective yellow. True yellow is such a narrow band that adding it to the mixture of red and green makes little difference.
  • RED lets through only RED, GREEN lets through only GREEN. So nothing gets through both, giving BLACK.
  • CYAN + MAGENTA - only TRUE BLUE gets through
  • CYAN + YELLOW - only GREEN gets through
  • MAGENTA + YELLOW - only RED gets through
  • MAGENTA + YELLOW + CYAN - transmits nothing (BLACK)
  • So Magenta, Cyan and Yellow are best for colour printing, Technicolour films, and water-colour paints. These operate by subtractive colour mixing.
  • All colour films have three colour layers made up of each of the secondary colours. You can show this by building up a layered picture on an OHP. Look at the negative and print produced by colour film and note the colours.
  • Pure yellow comes from a narrow part of the spectrum such as the light produced in a sodium flame. A red car near a sodium street lamp will look black.
  • The dyes in filters are used in water-colour paints, and even when the paints are mixed each paint does its own filtering job. When mixing blue and yellow in a paint box to make green, you are really using cyan and yellow, and the common colour transmitted is green.

This experiment was safety-tested in January 2007

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