A surgeon uses an endoscope to view inside a patient's body. The endoscope contains optical fibres made of glass. Explain how total internal reflection allows light to travel along a bent optical fibre and reach the surgeon's eye.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An endoscope is a flexible medical instrument that uses bundles of optical fibres to carry light into and out of the body. Each optical fibre is a thin rod of glass surrounded by a less optically dense cladding material.
Model answer (4 marks)
Light travels from the glass core, which is optically denser, into the surrounding cladding. At each core–cladding interface the angle of incidence must be greater than the critical angle, so the light is totally internally reflected. Because every reflection sends the light back into the glass, none escapes through the boundary. Repeating this reflection along the length of the fibre lets the light travel the full length even when the fibre is bent, delivering the image to the surgeon's eye.
Examiner tips
- Use the terms ‘core’, ‘cladding’, ‘critical angle’ and ‘total internal reflection’.
- Show the sequence: glass → cladding, incidence > critical angle, reflection back into glass.
- Explain that the repeated reflections keep the light confined along the bent fibre.
Common mistakes
- Confusing the core and cladding or saying light travels into the cladding.
- Forgetting to mention the critical angle or that all light is reflected back.
- Saying light is simply guided without referencing total internal reflection.
Mark scheme (4 marks)
- Light travels from a more optically dense medium (glass) into a less optically dense medium (cladding/air)
- The angle of incidence at the glass–cladding boundary must be greater than the critical angle
- All the light is reflected back into the glass / no light escapes through the boundary
- This reflection repeats along the length of the fibre, allowing light to travel the full length even when the fibre is bent
Key terms in this question
total internal reflection · optical fibre
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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