Explain why a layer of oil floating on water can produce a range of colours when viewed in white light, with reference to what happens at each boundary the light reaches.

Eduqas GCSE Physics — 5.2 Waves at material interfaces · Explain · 4 marks · View as Markdown

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

When white light shines on a thin layer of oil floating on water, bands of colour are visible across the surface.

Model answer (4 marks)

Some of the incident white light is reflected at the air–oil interface. The rest enters the oil and is reflected at the oil–water interface. The two reflected rays travel different optical paths, so when they recombine they interfere. For a given oil thickness the path difference is wavelength‑dependent, so constructive interference occurs for some wavelengths and destructive for others, giving a visible colour. As the oil thickness varies across the film, different wavelengths are reinforced, producing a range of colours.

Examiner tips

  • Use the phrase ‘interference of reflected rays’ to show understanding of the two boundaries.
  • Mention that the oil thickness changes the path difference, leading to different colours.
  • Show the sequence: reflection at top, transmission, reflection at bottom, interference.
  • Use UK spelling (interference, wavelength).

Common mistakes

  • Confusing refraction with reflection; forgetting that the reflected ray from the bottom boundary is the one that interferes.
  • Not recognising that the oil thickness varies across the film, so colours change.
  • Using vague terms like ‘bouncing back’ instead of ‘reflection at the interface’.

Mark scheme (4 marks)

  1. Some light is reflected at the air–oil boundary (top surface of the oil layer)
  2. Some light is transmitted into the oil and then reflected at the oil–water boundary (bottom surface of the oil layer)
  3. The two reflected beams overlap / combine / interfere
  4. Different wavelengths / colours of light are reinforced or cancelled depending on the thickness of the oil, producing different colours

Key terms in this question

boundary

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