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© Boardworks Ltd 2003
Waves: Total Internal Reflection and Communication
•Give the meaning of critical angle•Describe internal and total internal reflection•Recall and use the definition of refractive index n in terms of speed •Recall and use the equation sin i /sin r = n •Describe the action of optical fibres particularly in medicine and communications technology
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Reflected ray
Total Internal Reflection
Incident ray
Refracted ray
Angle i Angle r Refraction or Reflection?
15
30
45
60
75
Angle iAngle r
Angle r
At what angle of incidence did the ray change from refraction to reflection? It depends upon the material you used.
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Simulation of a ray of light passing through a semi-circular glass block
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Total Internal ReflectionThis angle is called the critical angle [ c]
i < c
Refraction
i = c
Critical case
i > c
Total Internal Reflection
[TIR]Different materials have different critical angles. Diamond has the lowest at 24º which is why it reflects so much light.
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The Critical Angle
How does refractive index affect the critical angle?
The greater the refractive index the smaller the critical angle.
Material Refractive index
Critical angle
Glass
Water
Diamond
42°
49°
24°
1.5
1.33
2.4
Research the missing values below and then make a conclusion…
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Total Internal Reflection
Optical fibres, that are used in communication, use total internal reflection.
A beam of light enters the optical fibre….
…NOTE how it is refracted as it enters the fibre……it travels down the fibre through repeated total internal reflections.
What are the applications of total internal reflection?
You could be asked to draw on the path of the beam in an exam.
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Optical versus electrical
Research why communications systems now use optical fibres instead of copper wires, use the table below to help you.
Optical Electrical
Information
Attenuation
Interference
Cost
Carry more Carry less
Less More
No Yes
More Less
How is attenuation solved in optical and electrical fibres?
Regenerators for electrical cables
Repeaters for optical fibres
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Digital and analogue
What is the difference between a digital signal and an analogue signal?
Digital signals can only be in one of two states, e.g. 0 or 1.
Analogue signals are a continuously changing variable.
What could they look like graphically?
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ADVANTAGES
Signals are clearer less susceptible to noise.
Can be used quickly by computers.
Carry digital signals using electromagnetic waves which travel at the speed of light.
Carry much more information.
Digital hardware is much smaller.
Easier to send over long distances.
DISADVANTAGES
Digital hardware is expensive at the moment.
Although digital signals are unaffected by electrical interference, they don’t give a complete signal [just lots of samples] - some people feel that analogue vinyl records sound better than digital CDs for this reason.
What is noise?Noise is any unwanted
information.Examples of noise are?Electrical storms and random thermal noise.
Which signal is most prone to noise?
AnalogueWhich signal carries the most information?
Digital
Digital compared to Analogue
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Communication systems
Communication systems include the same basic components. You need to be able to define/describe the following:
Encoder
Amplifier
Decoder
Modulator
Transmitter
Receiver
Transducer
Storage
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Definitions
Component FunctionEncoder
Decoder
Modulator
Transmitter
Receiver
Storage
Transducer
Amplifier
Changes information into readable form
Allows wave to carry impulses AM/FM
Changes information to original form
Stores information e.g. CD, DVD, tape
Makes oscillations
Collects information
Changes information into electrical form or the other way round (microphone, speaker)
Increases intensity of received waves
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Which of the following is not a use of total internal reflection?
A. Periscopes
B. Endoscopes
C. Fibre optic communication
D. Mirrors
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What does attenuation mean?
A. Strengthening
B. Increasing
C. Larger
D. Weakening
