Explain why a material that appears red absorbs some colours of visible light but not others, and describe what happens to the energy of the absorbed light.

Edexcel GCSE Physics (1PH0) — 4.1 Waves · Explain · 4 marks · View as Markdown

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

When white light, which contains all the colours of the visible spectrum, strikes the surface of a red object, the object appears red to an observer.

Model answer (4 marks)

Only the red wavelengths are reflected by the surface of the object. All other visible wavelengths are absorbed by the material.
The absorbed wavelengths have frequencies that match the energy gaps between electron energy levels in the material, so the electrons absorb the light energy.
The absorbed energy is subsequently released as heat (thermal energy) rather than as light.

Examiner tips

  • Use the word "reflect" for the colour that appears, "absorb" for the others. Mention the electron energy level match. State the energy is released as heat, not light.
  • Be concise – 4 marks, so keep each point brief but complete.

Common mistakes

  • Saying the object absorbs all colours and reflects none. Confusing reflected light with transmitted light. Claiming the absorbed energy is re‑emitted as light instead of heat.

Mark scheme (4 marks)

  1. Only red light is reflected (by the surface of the object)
  2. All other colours/frequencies of visible light are absorbed (by the object)
  3. The frequency of those absorbed colours matches the energy levels of the electrons (in the material), so the electrons absorb the light energy
  4. The absorbed energy is re-emitted over time as heat/thermal energy (rather than as light)

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