Catholic High 2004 Prelims P2 Q7 (adapted)
The figure below shows a ray of light PQR passing along a simple optical fibre to its end at R.
(a) Explain why the ray PQ does not leave the optical fibre at Q.
(b) Describe what happens to the ray at R.
(c) State one advantage of optical fibres rather than copper wires for carrying telephone communications.
(d) Explain why radio waves are used to send telephone messages over long distances and are usually sent from large aerials on top of high buildings or towers.
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Answer:
(a) The angle of incidence at Q is greater than the critical angle of the optical fibre (usually 42° for glass to air). Since the ray is travelling from a denser medium to a less dense medium, total internal reflection occurs. In total internal reflection, the ray does not leave the optical fibre at Q.
(b) The angle of incidence at R is 15°. This is smaller than the critical angle, and hence, the ray emerges into the air and is bent away from the normal with an angle of refraction greater than 15°.
(c) It is more efficient as less energy is lost as compared to copper wires (copper wires have resistance).
(d) Radio waves have very long wavelengths and thus, can travel around due to refraction through different layers of the atmosphere.
Using large aerials on top of buildings reduce the change of reflection and interference (to and from the atmosphere).
