Questions:
LEVEL: AS
PHYSICS – Waves and Oscillations
11. Total internal reflection and fibre optics (15 minutes)
The Scientific Times
Most of us are aware that doctors often use fibre optics for diagnosis and treatment. Endoscopes consist
of a bundle of optical fibres encased in a flexible protective covering. Endoscopy can offer doctors an
opportunity to examine internal organs. It is also used in 'keyhole' surgery. Nowadays, however, fibre
optics are being used in an ever-increasing variety of applications. They are even being used to treat
'sick' buildings, aircraft and ships.
When humans become ill or are injured, we are prompted to seek help by the pain we feel. Until
recently the only way to check on the 'health' of buildings, aircraft and ships was by regular checks as
part of a maintenance programme. This situation is now changing. A good example to illustrate this
point can be found in the aircraft industry.
Each time a jet aircraft is operated its engines are subjected to intense temperatures. When the aircraft
lands and the engines are turned off, they cool and are left to await their next application. Engineers are
fully aware that tiny cracks, too small to be seen by the human eye, can develop during the aircraft's
working life. The cracks are quite likely to develop deep within the structure of the engine, far from the
attentive eyes of the engineers. However, what can start as a tiny crack has the potential to grow into a
much more serious threat to safety. Regular and painstaking maintenance checks, using radioactive
materials and photographic film, have been used to detect the presence of such faults.
Aircraft manufacturers are now making more and more use of advanced composite materials such as
carbon fibre reinforced plastic. It is hoped that, in the future, aircraft will be built with tiny sensors
built into their very structure. Optical fibres could be embedded within the fabric of the aircraft. The
fibres could be sensitized at carefully selected points along their lengths. When the fibres are stretched
or compressed, they would actually change colour at these points according to the strains they were
exposed to. As the composite materials presently used contain many fine strands of glass, the
introduction of optical fibres would largely go unnoticed.
The tiny sensors buried throughout the body of the aircraft would operate as an artificial nervous
system. A map of the stresses and strains experienced by the aircraft could be built up from the
information they yield. The aircraft could, in effect, examine itself and inform the engineers as to what
work should be carried out before its next flight. The aircraft really would be a 'smart structure'.
(a) The diagrams below show three possible angles of incidence for a ray of light approaching a glassair boundary. Show what happens to the ray in each case.
(3 marks)
Copyright © Pearson Education Limited 2001
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Questions:
LEVEL: AS
PHYSICS – Waves and Oscillations
(b) What is meant by 'total internal reflection'?
When a ray of light approaches an optically less dense material at an angle of incidence greater than the critical
angle, it is entirely reflected back from the boundary. This is called total internal reflection.
(c) Complete this sketch a diagram of a fibre optic to show how light travels through it, undergoing total
internal reflection as it goes.
(3 marks)
(d) Using the information in the passage above, describe one advantage of smart structure technology
as applied to aircraft maintenance.
(2 marks)
It is hoped that the smart structure described in the passage will be able to help predict when the structure will
next need attention.
(e) Modern aircraft rely heavily on electrical machines, are guided by radar and are expected to fly safely
in a variety of weather conditions, e.g. lightning. Bearing these facts in mind, suggest an advantage that
optical fibre sensors would offer modern aircraft.
(2 marks)
Optical fibres are not affected by electrical interference. Their use would not be hampered by any of the
conditions suggested in the question.
(f) Suggest one non-safety-related advantage of the use of advanced structural health monitoring
systems such as the one outlined in the passage.
(2 marks)
Conventional maintenance procedures can result in the machine or structure being out of action for long periods
while checks are carried out. As smart structures are designed to continually check for problems they could help
reduce running costs.
(Total marks 15)
Copyright © Pearson Education Limited 2001
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