Light and Sound
5 marks · NormalOptical fibres use total internal reflection to transmit light signals long distances, which is similar to how prisms are used in periscopes.
Explain why total internal reflection occurs inside an optical fibre and draw a labelled diagram to illustrate it.
State one medical and one communication use of optical fibres.
A certain type of glass has a refractive index of 1.56. Calculate its critical angle for total internal reflection.
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.
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(a) — · must show diagram showing fibre core and cladding, ray hitting interface at >critical angle, ray stays inside
(b) Medical: endoscopes; Communication: internet/telephone/data transmission · must show one valid medical, one valid communications use
(c) 40° degrees · must show sin c = 1/n, final value 40°
Must show: Diagram with angles labelled, core and cladding, rays undergo reflection at interface, Correct formula use for critical angle
Also accept: Endoscopy for medical, internet/telephone for communication
Common mistakes: Using wrong formula for critical angle; giving wrong examples of fibre use
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correct diagram with core, cladding and ray reflected at interface with angles labelled.
B1 (a) · 1 mark · Explanation refers to angle > critical angle and higher refractive index in core.
B1 (b) · 1 mark · One valid medical use (e.g., endoscopes).
B1 (b) · 1 mark · One valid communication use (e.g., fibre-optic internet).
M1 (c) · 1 mark · Correct method for critical angle, final answer within 1° of 40°.
Worked solution
a) Total internal reflection occurs when light in the fibre core (higher refractive index) strikes the boundary with the cladding (lower index) at an angle greater than the critical angle, so all the light is reflected inside.
Diagram: Draw fibre cross-section, mark core, cladding, ray at angle>c, normal at interface, reflected path.
b) Medical use: endoscopes for viewing inside the body. Communication use: fibre-optic cables for internet/telephone.