Class practical

Think of colour as more than simply a property of light – think of it as the result of our brains interpreting messages from our retinas. Those messages are produced when receptors in the retina are stimulated by coloured light.

Thinking about colour in this way can make it easier to explain the observations made during the investigations below – and to explain the results of any colour perception anomalies experienced by your students (so-called 'colour blindness').

Lesson organisation

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You could use the slide presentation  Seeing in colour (488 KB) (or printed cards of the images) for a short starter or plenary activity about observation – to make the point that we see colour differently in different situations. What we see on the white screens or cards (as an ‘after-image’) depends on what we were looking at before. Alternatively, you could use the images with the student sheet to try to gather information about experiences within the group, and use that information to develop hypotheses about colour vision or to evaluate the theory of trichromatic colour perception.

Apparatus and Chemicals

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Health & Safety and Technical notes

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There are no safety issues with this practical. If any students become stressed by realising that their perception of colour is different from others, you would need to take action to reduce that stress.

1 Students will need one white card in each set of cards, if you are not using the slide presentation which includes blank white slides.

Ethical issues

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It is important to present this topic with care. Some students may discover for the first time that their colour perception is different from others. Avoid using the word ‘normal’ to describe ordinary trichromatic colour vision. Make it clear that differences in colour vision and colour perception are very common (for example, 8% of Caucasian males have unusual variations in red and green colour vision). Present the variations as points on a line of continuous variation rather than defects. It is very difficult to be sure how anyone else perceives the world in any case, and differences from one to another may simply be explained by differences in how people describe what they perceive.

It is not easy to make deductions about heredity of colour vision. Although red-green colour vision is a sex-linked characteristic (the genes are carried on the X-chromosome), unusual variants of red and green colour-detecting pigments can be formed by crossing-over and recombination during meiosis – so many individuals have colour perception very different from that of either of their parents. Blue-cone pigments are coded by a gene on an autosomal chromosome and show more familiar patterns of simple inheritance, but are very rare. If students discover their colour perception is different from their parents, that observation should not be linked to heredity.

Procedure

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SAFETY: There are no safety issues with this practical. Be aware of any students becoming stressed by the experience and react appropriately.

Preparation

a Print (and laminate) the cards if preferred to the slide presentation.

b Locate (and laminate) suitable printed colour photographs for the extension work.

Investigation

c View the coloured images as a projected presentation or from printed cards.

d Record the colours seen by individuals, by most of the group, and by some of the group.

e Discuss the observations.

f Attempt to build a hypothesis to explain the observations.

g Use the observations to evaluate a presented hypothesis about three kinds of colour receptors in the retina.

Teaching notes

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Many investigations of colour, such as the effects of mixing light from two sources, or viewing coloured items under coloured light, are presented as properties of the physics of light. In fact, they usually depend on the biology of colour perception.

In our retinas, there are three kinds of receptors. Each absorbs and responds to a different range of wavelengths of light. (See Wikipedia graph of absorption spectra) These are usually described as red, green and blue receptors, although they absorb light in overlapping ranges of wavelengths. When any receptor is stimulated, it sends a message to the optical centre of the brain and our brains interpret the combination of incoming messages as colours in what we see. For example, yellow light stimulates both red and green receptors. When both red and green receptors are stimulated, we interpret what we see as yellow.

These three colour receptors correspond to the colours produced by television screens. A television picture is made of a series of pixels that emit red, green or blue light. When we look at the screen, our brains interpret the mixture of red, green and blue as colours across the whole spectrum. Similarly, when you select a colour from the custom area of most Microsoft packages, the colour is described in terms of its relative proportions of red, green and blue.

When we stimulate any individual receptor for a period, and then cease stimulation (for example by looking at a red shape for a while and then a plain sheet of white paper), our brain reacts as if it were ignoring the message from the red receptor. Therefore we see an after-image which matches what we would see (in this case) if both the green and blue receptors were stimulated (and red no longer stimulated). This colour is sometimes called the complementary colour for red. In this case, cyan (greeny blue) is the complementary colour for red. Yellow is the complementary colour for blue, and magenta is the complementary colour for green.

Cyan, yellow and magenta (with black) are the commonly used colours for inks to print colour images. Look at a newspaper photograph under moderate magnification to see dots of these colours (and look at the edge of the newspaper to see ink test patches for the printer). CMYK – cyan, magenta, yellow and 'kohl' (which is black) – is another way of defining colours for screen images or print. Cyan stimulates both blue and green receptors. Magenta stimulates both red and blue receptors. Yellow stimulates both green and red receptors. The amount of kohl in the mix determines the brightness of a colour.

'Colour-blindness' does not simply mean 'not able to distinguish colour'. Often it is more an issue of discriminating two colour in similar tones, when they are right next to each other, or mixed up as in the Ishihara colour vision tests. Someone who is red-green 'colour-blind' will often be able to identify blocks of green or red and will know that grass is not the same colour as UK post boxes, but may not be able to see dots of green amongst red and vice versa.

Health & Safety checked, August 2010

Downloads

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Download the student sheet  Investigating how we see colour (66 KB) with questions and answers.
Download Seeing in colour (488 KB).

Web links

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http://www.toledo-bend.com/colorblind/Ishihara.asp
View the Ishihara colour blindness tests on-line. Be aware that the combination of your computer set up, screen set up and projector may distort the colours viewed.

http://www.opticien-lentilles.com/daltonien_beta/nouveau_test_daltonien.php
This site is authored by a French optician and includes a nice test of colour vision. The translation is occasionally weak, but the site is comprehensible and interesting. (

http://www.colblindor.com/
This is a fascinating and comprehensive site about colourblindness developed by Daniel Flück, who is himself colour-blind. One intriguing element of the site is a colour-blindness simulator (www.colblindor.com/coblis-color-blindness-simulator) which allows you to view interesting photographs or upload your own (if 600k or less) and see how they might look to someone with different forms of colour-blindness.

(Websites accessed October 2011)