Total Internal Reflection
3 marks · EasyOptical fibres are used to carry communication signals as pulses of light.
State the role of total internal reflection in an optical fibre.
Explain, using the idea of critical angle, how the light pulse remains inside the fibre.
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(a) It keeps the light signal inside the fibre by reflecting it at the boundary.
(b) The light pulse hits the boundary at an angle greater than the critical angle, so total internal reflection occurs and the pulse stays within the fibre.
Common mistakes: Not mentioning critical angle.; Not explaining that light remains trapped by repeated reflection.
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Marking points
B1 (a) · 1 mark · States total internal reflection keeps light inside the fibre.
B1 (b) · 1 mark · Refers to angle of incidence being greater than the critical angle.
B1 (b) · 1 mark · Explains this causes total internal reflection so the pulse does not escape.
Worked solution
a) Total internal reflection keeps the light pulse inside the optical fibre so it can travel long distances without loss.
b) The light hits the boundary inside the fibre at an angle greater than the critical angle. This means that instead of passing into the surrounding air or cladding, it is totally internally reflected and bounces along the fibre.