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Infra ­ Red Signals

Infra ­ red radiation can be used in :

Remote controls
Burglar alarms
Automatic doors
Dusk lights ( sensors )

In communication two types of signals are used :


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Analogue signals

Analogue is a variable
Can be distorted by interference

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Digital signals

Digital signals can be either on or off

They are much more useful and efficient because :

There is little or no interference
The signal can be ` cleaned up '
Many signals can be transmitted at once ( multiplexing )

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Light rays

Firstly we need to understand how light travels through
When light passes from one medium to another its speed

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As the angle of incidence ( ray of light going in ) gets larger
and larger, the amount of refraction also becomes larger

Eventually the immerging ray of light lies parallel with the glass
this is called the critical angle

If the angle becomes larger than the critical…

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Optical fibres use total internal reflection ( TIR ) to
transmit signals, usually Infra ­ red ( IR ) or Ultra Violet
( UV )
This allows rapid transmissions and multiplexing


Optical fibres are being used to replace copper wires in
the telephone system
The telephone conversations are sent…

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Infra ­ red rays are often used
At the other end the signals are converted back to
electricity by a photo ­ diode which acts rather like an
LDR ( Light ­ Dependent Resistor )

Over 10000 telephone conversations can be carried at the
same time by one fibre

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