A student is standing in a swimming pool. The student notices that when they look at their legs through the water, their legs appear to be in a different position from where they actually are. Explain why this happens in terms of what occurs when light travels from water into air.

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).

Model answer (4 marks)

Light travels more slowly in water than in air. When it reaches the water–air interface it is refracted – its direction changes. Because the speed in air is higher, the ray bends away from the normal. The brain then traces the bent ray back in a straight line, so the legs appear to be in a different, shallower position.

Examiner tips

  • Use the word "refracted" and mention the change of speed
  • Explain the bending away from the normal
  • State that the brain traces the light in a straight line
  • Link the apparent position to the shift caused by refraction

Common mistakes

  • Confusing refraction with reflection
  • Saying the light bends towards the normal
  • Not mentioning the change in speed or the brain’s straight‑line tracing

Mark scheme (4 marks)

  1. Light changes speed when it moves from water to air
  2. Light is refracted / changes direction at the boundary between water and air
  3. The light bends away from the normal as it passes from water into air
  4. The observer's brain traces the light back in a straight line, so the legs appear to be in a different (shallower/shifted) position

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