Total Internal Reflection
3 marks · NormalA laser beam is sent into an optical fibre made of glass surrounded by air. Explain why the light remains trapped inside the fibre when it travels around bends.
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Also accept: Light always strikes boundary at > critical angle; Total internal reflection at glass–air interface
Common mistakes: Saying 'the light bounces' but not referencing TIR; Not mentioning angle or refractive index
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Marking points
B1 · 1 mark · States total internal reflection occurs at glass–air boundary
B2 · 1 mark · Explains the angle of incidence is greater than the critical angle (so light remains inside)
B3 · 1 mark · Relates principle to light remaining within the fibre, even on bends
Worked solution
Light inside the optical fibre always hits the glass–air boundary at an angle greater than the critical angle, so total internal reflection occurs. This means the light keeps reflecting inside the fibre rather than escaping. Bends in the fibre still allow for this as long as the angle stays above the critical angle, so the light remains trapped and travels along the fibre.