Optical Fibres
3 marks · NormalExplain how total internal reflection inside an optical fibre ensures that light follows the path of the fibre even if it bends.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
Also accept: Light striking the core–cladding boundary at angles greater than the critical angle stays inside; this repeats as the fibre bends; light remains trapped in the core.
Common mistakes: Omitting the role of the critical angle or how reflection happens at curves.
Show mark scheme and worked solution
Marking points
M1 · 1 mark · Mentions light is reflected at the boundary between core and cladding.
M1 · 1 mark · States that the angle of incidence inside the fibre is greater than the critical angle (or always sufficient for TIR).
A1 · 1 mark · Explains that therefore the light keeps reflecting and follows the curve of the fibre.
Worked solution
When light in the fibre reaches the core–cladding boundary at an angle greater than the critical angle, it is totally internally reflected. This process happens repeatedly, so even if the fibre bends, the light continues to reflect off the boundaries and stays within the core, following the shape of the fibre.