A market stall sells oranges under a sodium street lamp that emits only yellow light. Explain what colour the orange skin appears under the yellow light, and explain what colour the orange skin would appear under blue light only.

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)

Under the sodium lamp the only light present is yellow. The orange skin reflects the yellow wavelengths that it receives, so it appears yellow to the observer.

If the only light available is blue, the orange skin does not reflect that wavelength. It absorbs the blue light, so no light is reflected and the skin appears black (or very dark).

Examiner tips

  • Use the command word ‘explain’ – describe the process of reflection/absorption and state the resulting colour.
  • Show the two separate situations (yellow light, blue light) clearly to earn all marks.
  • Use the exact terminology: ‘reflects’, ‘absorbs’, ‘appears’.”]
  • common_mistakes

Mark scheme (4 marks)

  1. Under yellow light, the orange skin reflects yellow light (because orange surfaces reflect red and yellow wavelengths)
  2. So the orange skin appears yellow (under the sodium lamp)
  3. Under blue light only, the orange skin absorbs the blue light (because orange skin does not reflect blue wavelengths)
  4. So the orange skin appears black (under blue light only)

Key terms in this question

colour

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