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Hue and eye

Nick Selley shows how the science of colour can be made accessible to primary pupils.

Colour is an excellent topic for primary work on the processes of science - observation, comparing and planning investigations:

* it is familiar to children through daily life, clothes, art, and beautiful natural phenomena such as sunsets and flowers * the necessary resources (for example, paints, torches, and coloured card) are cheap, safe and easily obtained * perhaps perversely, the absence of national curriculum references at this level leaves the field open for pupils' investigations, since their discoveries will not be constrained by any anxiety about "right answers".

Primary pupils' experiences will eventually contribute to the knowledge that may be tested in the key stage 3 SATs, or at GCSE - but that is far enough ahead not to overshadow the primary school or Year 7 work. Nothing is more hostile to children's genuine science activity than the half-hidden pressure on them to adopt advanced theoretical explanations. In primary science especially, the observable facts are often quite straightforward, but the textbook explanations require the kind of understanding associated with level 6 or 7, and are therefore incomprehensible to most eight to 11-year-olds.

The KS3 programme of study includes knowledge of "how coloured objects appear in white light and in coloured light". As a visible effect this is quite easily investigated by any pupil with the procedural skills of systematic observation, recording and hypothesising. With a dark corner (or cardboard box) and a sturdy lamp or lantern, primary pupils from eight years old could investigate some old favourites such as red lettering on white paper, seen by red and then green light; or a red geranium with green leaves (real or pictorial) viewed in different lights. Does the school dinner look more or less appetising under blue light? And so on.

Soon, when these effects have lost their surprises, at least some of the pupils will ask "Why?" and "How do colours work?". It might seem time to introduce the theory that white light consists of many colours (and the question of how many), and that while white surfaces can scatter all these, coloured surfaces absorb most colours and send back only the coloured light that we see. For example, a red surface absorbs all the colours except red. This theory is to be found in many books on light aimed at junior pupils.

However, we are assuming that the pupils already know and accept the theory that we see objects by reception of light scattered from these objects, but there is strong evidence that most children (more than 80 per cent at age 11) believe nothing of the sort. To them it seems obvious that we see an object by means of something sent out from our eyes towards the object. Anyone peering about, straining to see something, feels the active effort involved, and will be convinced that the eyes are initiating the act of sight. Of course, some external source of light is necessary for vision to occur, but the thoughtful child will recognise that there has to be collaboration between the illumination and the optical ray, which both reach the object together.

This might seem an insuperable obstacle to a pupil's understanding of colour vision but, luckily, it is not. Many children are willing t accept that a surface can, when illuminated, send out its colour. They assume that our eyes, when using their "sight" power to see the world around, would see only black-and-white shapes if it were not for this added colour sensation. Some children have suggested that the optical beam from our eyes reaches the illuminated object and then returns, bringing back both the image shape and its colour.

Pupils can be shown how red light can change a green leaf into a dull brown leaf. Children can accept that this is an optical illusion - the leaf is restored to green as soon as we allow normal daylight on to it again - because the world is full of optical illusions (for example, that there people inside the television). We can work out with them the rules governing the effect: white light can activate all surface colours (except perhaps black). A coloured light, say red, activates a red response both from a white surface and from a red one; but red light has a subdued, disappointing effect on green or blue surfaces, and almost completely robs them of their colour. So "White Light Rules" and for all the rest, the colour of the light and the colour of the object have to match.

One question that can arouse warm debate among pupils: Is black a colour? You can have black objects, but apparently not black light. (Unless, of course, that is what darkness is! As one pupil saw it, shadows are caused by black light reflected off an object.) Colours can be close, or opposite. This can be investigated by systematic paint mixing. Try mixing red and yellow paint in various measured proportions (there is a maths bonus here) from (9R plus 1Y) to (5R plus 5Y) and on to (1R plus 9Y). the pupils will observe shades of orange, but never a hint of green, or violet. So red is close to orange, but opposite to green. Try mixing these opposites, and they can see how they get a drab, almost colourless muddy brown. Get them to stare at a green shape for a minute, then transfer their gaze to a neutral-coloured wall or ceiling: they will see pink monsters. The battle of the colours can be quite dramatic.

With no more theory than the above, primary pupils can learn, and understand, that all colours (hues) or inks, dyes and paints can be made by mixing no more than three basic colours: magenta, cyan and yellow. A similar rule applies to television-screen pictures, which are composed of tiny patches of three colours, except that these are red, green and blue. It seems that our brains can combine three colours, in various proportions, to give the effect of many other colours. This helps show pupils how our eyes do not need to have hundreds of different colour receptorsrecognisers - they manage with just three sorts, and a clever brain to "process the data".

A problem for investigation: could you devise a colour printer that can print black without using black ink?

More questions for discussion:

* do different people see the same colour when they look at the same thing?

* Does anyone really dream in colour, or do they add the colours later, by imagination? How could you test this?

* Are soap-bubble or oil-film colours the same as, or completely different from, any of the colours in the spectrum of the rainbow?

Nick Selley is a lecturer in the school of education at Kingston University

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