Explain why a fish swimming underwater is able to see objects above the water surface only within a limited cone of view, even though objects exist all around above the surface.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Light travels at different speeds in water and in air. When light passes from one medium to another, its direction can change at the boundary.
Model answer (4 marks)
Light is refracted when it passes from air into water because the speed of light changes at the interface.
When light travels from water to air, if the angle of incidence exceeds the critical angle it is totally internally reflected and cannot escape into the air.
Only light that strikes the surface at an angle smaller than the critical angle can pass into the water and reach the fish’s eye.
Consequently the fish’s view of the world above the surface is confined to a narrow cone directly overhead, limiting what it can see.
When light travels from water to air, if the angle of incidence exceeds the critical angle it is totally internally reflected and cannot escape into the air.
Only light that strikes the surface at an angle smaller than the critical angle can pass into the water and reach the fish’s eye.
Consequently the fish’s view of the world above the surface is confined to a narrow cone directly overhead, limiting what it can see.
Examiner tips
- Use the term ‘refracted’ and mention the change of speed; cite the critical angle and total internal reflection; explain that only light below this angle reaches the eye; describe the resulting narrow cone of view.
Common mistakes
- Confusing refraction with reflection; not mentioning the critical angle; saying the fish can see all around because light bends; ignoring that only light below the critical angle reaches the eye.
Mark scheme (4 marks)
- Light is refracted / changes direction when it passes from air into water (or water into air) because it changes speed at the boundary
- Light travelling from water to air at an angle greater than the critical angle undergoes total internal reflection and cannot reach the fish's eye
- Only light that strikes the water surface at an angle less than the critical angle can pass through into the water and reach the fish
- This means the fish's entire view of the world above the surface is concentrated / compressed into a narrow cone directly above, limiting what the fish can see
Related
- All Eduqas GCSE Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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