A stage lighting designer shines a green spotlight and a red spotlight onto the same area of a white stage floor. Explain what colour the overlapping area appears and why, and also explain what colour a blue costume worn by a performer standing in only the red spotlight would appear.

Eduqas GCSE Physics — 6.4 Colour and frequency · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The overlapping area appears yellow because the green and red lights mix to give yellow light, and a white surface reflects all wavelengths, so the yellow light is seen.
A blue costume in only the red spotlight appears black because the blue fabric absorbs the red light and no blue light is present to be reflected.

Examiner tips

  • Use the word ‘mix’ for colour addition, mention the white surface reflects all wavelengths, state the blue fabric absorbs red, and note absence of blue light makes it black
  • Keep answer concise and use exam‑specific terms like ‘yellow light’ and ‘black’

Common mistakes

  • Claiming the overlapping area is white instead of yellow
  • Saying the blue costume appears blue in the red light
  • Forgetting to explain that the blue fabric absorbs red light

Mark scheme (4 marks)

  1. Green light and red light mixing together produce yellow light
  2. White surfaces reflect all colours of light, so the overlapping area reflects the yellow (combined) light to the eye
  3. The blue costume absorbs red light (red light is not reflected by the blue fabric)
  4. No blue light is present in the red spotlight, so the costume appears black

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