A swimming pool appears shallower than it actually is when viewed from above the water surface. Use your knowledge of refraction to explain why the pool appears shallower than its true depth.

Eduqas A-Level Physics — 2.6 Refraction of light · Explain · 5 marks · View as Markdown

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

Light from the bottom of a swimming pool travels upward through the water and then passes into the air above.

Model answer (5 marks)

Light travels from the water into the air at the water surface. The ray bends away from the normal as it enters the less dense medium. The observer’s eye traces the refracted ray back in a straight line. This straight‑line trace meets the bottom of the pool at a point that is higher, i.e. closer to the surface, than the actual bottom. Therefore the pool appears shallower than its true depth.

Examiner tips

  • Use the word ‘bends away from the normal’ to show understanding of refraction direction.
  • Explain the eye’s straight‑line trace to link refraction with apparent depth.
  • Mention the less dense medium (air) to justify the bending direction.
  • Conclude that the apparent depth is less than the true depth.

Mark scheme (5 marks)

  1. Light travels from water into air (at the water surface / boundary)
  2. Light changes direction / bends away from the normal as it enters the less dense medium (air)
  3. The observer's eye traces the refracted ray back in a straight line
  4. This straight-line trace meets the bottom of the pool at a point that is higher / closer to the surface than the actual bottom
  5. Therefore the pool appears shallower than its true depth

Key terms in this question

refraction

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