A swimming pool appears shallower than it actually is when viewed from above the water surface. Explain why the pool appears shallower than its true depth.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When a lifeguard looks straight down into a swimming pool, the bottom of the pool appears to be much closer to the surface than it really is.
Model answer (5 marks)
Light from the bottom of the pool travels through water and reaches the water–air interface. At this boundary the light is refracted – it changes direction. Because water is denser than air, the ray bends away from the normal as it enters the air. The observer’s eye then follows the refracted ray back in a straight line, giving the impression that the source of the light is higher up. Consequently the bottom of the pool appears closer to the surface, so the pool looks shallower than its true depth.
Examiner tips
- Use the term ‘refracted’ and mention the water–air interface
- Explain the direction of bending (away from the normal) and why this makes the bottom appear higher
- Show the chain of reasoning from refraction to apparent depth
Common mistakes
- Failing to mention the water–air boundary or refraction
- Confusing the direction of bending (bending towards the normal)
- Not linking the refracted ray to the apparent position of the bottom
Mark scheme (5 marks)
- Light travels from the bottom of the pool through water into air (at the water surface)
- Light changes direction / is refracted at the boundary between water and air
- As light moves from water (denser) into air (less dense), it bends away from the normal
- The observer's eye traces the refracted ray back in a straight line, so the apparent source of light appears higher / closer to the surface than the actual source
- This makes the bottom of the pool appear closer to the surface / the pool appear shallower than it truly is
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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