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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 :

Analogue
Digital

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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
glass
When light passes from one medium to another its speed
changes

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Angles

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




Communications

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
Fibre…

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