White Light & Colour

For ages 11–12, build understanding that white light is a mixture of all visible colours (ROYGBIV ordered by frequency), that a prism disperses white light because different frequencies refract by different amounts (violet bends most, red least) and a second prism recombines it, that objects appear coloured by selective absorption and reflection (so a red object looks red in white light but near-black in pure blue light), and that filters subtract rather than add colour. The pivotal learning move is predict-observe-reconcile: a learner predicts and explains an object's colour under coloured light, then watches it change, which refutes colour-as-innate-property. Subtractive reasoning is documented as genuinely hard, and additive light mixing must be kept visibly distinct from subtractive pigment/filter behaviour.

• Colour is an innate, fixed property of the object (a red dress 'just is' red), independent of the light illuminating it.
• The light merely 'brightens' the object's own colour, rather than the object selecting some frequencies from the light and absorbing the rest.
• Colour is ADDED by the light or a filter 'dyes' all light its own colour (adder), rather than SUBTRACTED by selective absorption.
• White light is 'just light' / 'normal light', not a mixture of all colours — which undermines both dispersion and colour.
• Confusing additive light mixing (red + green LIGHT = yellow) with subtractive pigment/filter behaviour; e.g. expecting red + green spotlights to make brown (documented 70%+ error).
• Believing a prism 'adds' or 'creates' colours, rather than separating colours already present in white light.
• Predicting a red object under blue light stays red or looks blue/dark-red, instead of black (no red present to reflect).
• Forgetting the whole path — perceived colour is the light REACHING THE EYE, which depends on BOTH the pigment and the illuminating light.

• A prism dispersing white light into ROYGBIV (violet refracts most, red least) — paired with a SECOND prism recombining the spectrum back to white.
• The visible spectrum ordered by frequency/wavelength (red → violet), labelled with the physical quantity, not just colour names.
• Selective-absorption diagram: white light (all colours) strikes a red apple; most colours absorbed (greyed), red reflected to the eye.
• The 'same object, different light' comparison: a red object in white light (red) vs the same object in blue light (black).
• Filter-as-subtractor diagram: white light through a red filter — red transmitted, the rest absorbed — with a coloured object shown as itself a selective filter.
• The IOP Wrong Track / Right Lines paired statements as a refutation representation.

• Use the IOP 'Wrong Track / Right Lines' refutation explicitly: name the intuition ('the dress is red and the light just brightens it up') and contrast it with the model ('white light falls, the dress absorbs most frequencies and reflects red to your eye') — a refutation-text move that outperforms simple telling.
• Secure 'what IS white light' FIRST (Haagen-Schützenhöfer): pupils conceive white light as 'normal light', which blocks colour understanding — use a prism to split white light AND a second prism to RECOMBINE it, so white = all colours together is seen, not asserted.
• Run predict-observe-reconcile with coloured light: pupils predict, then WATCH a red object go black under blue light (and a white object take the lamp's colour) — the observation directly refutes colour-as-innate-property.
• Confront adder-vs-subtractor head-on (IOP 'Understanding colours'): ask 'does the red filter turn all the light red, or just cut out everything except the red already in white light?' — establish filter = selective SUBTRACTOR, and a coloured object = a selective filter.
• Sharply separate SUBTRACTIVE pigment/filter behaviour from ADDITIVE light mixing (red + green LIGHT = yellow) — the two are opposite and routinely conflated (70%+ of students wrongly expect red+green spotlights to make brown); teach objects and filters as subtractive.
• Order the spectrum by a PHYSICAL quantity (frequency/wavelength), not just 'rainbow order' — red = lowest frequency/longest wavelength through to violet = highest — tying colour to the wave property that dispersion sorts.
• Present 'black under blue' as the idealised/limiting case: use saturated pigments and reasonably pure coloured light for a clean effect, and acknowledge real objects look 'very dark', not perfectly black.
• Build in accessibility (NASA JPL): for colourblind pupils, label colours with numbers/legends so the assessed reasoning is the absorption model, not hue discrimination.