Compare coloured light with coloured materials. Explain why mixing lights differs from mixing paints.
Newton’s prism work in the history notes is the starting point for this extension.
A prism can spread white light into a visible spectrum because different wavelengths refract by different amounts. The spectrum is continuous; named colour bands are a convenient description, not hard boundaries in nature. Visible light is only a small part of the electromagnetic spectrum.
A red object under white light reflects red wavelengths more strongly than many others. Under light with little red content, it can look dark. The object does not necessarily produce red light. Transmission through a coloured filter also selectively changes the light that reaches the observer.
Ideal red, green and blue light channels can combine to produce many perceived colours. Red plus green gives yellow; green plus blue gives cyan; red plus blue gives magenta. All three at suitable balanced intensities appear white. This is an RGB display model, not a claim that the light has become one new wavelength.
Pigments absorb parts of the light illuminating them. Mixing pigments changes which wavelengths remain after absorption and scattering. A recipe for mixing beams of light should not be used as a recipe for mixing paint. Human colour perception also depends on the visual system and context.
Relate visible colour to light reaching the eye and distinguish additive light mixing from selective absorption.
Physics Girl · Demonstration
Explain why an object’s colour depends on both the incoming light and the observer, rather than a colour label alone.
Compare RGB light mixing with a single light source and a coloured filter.
Predict red plus green, then test it. In the single-bulb view, explain which light a filter transmits and which it removes.
Sketch one before-and-after comparison. Label what you changed and what stayed the same. Use the lesson’s vocabulary to explain the result.
Ideal additive RGB display model. The swatch represents a perceived mixture, not a single wavelength.
Red and green light together produce which perceived colour in the ideal RGB model?
This is light mixing, not paint mixing.
The additive combination is yellow.
Does a red object necessarily emit its own red light? State yes or no and explain your reasoning.
Separate a source from a reflecting object.
It can appear red by selectively reflecting incoming light.
Predict a red object’s appearance under only blue illumination.
An ideal red surface that absorbs blue would appear dark. Real materials are not perfect filters, so the exact appearance depends on their reflectance and the illumination.
The incident white light already contains a range of wavelengths. The prism separates their paths.
Commit to an answer, then use the feedback to check your reasoning.
