P1E REVISION NOTES

SINGLE SCIENCE, OCR BOARD - HIGHER TIER, PHYSICS - P1E

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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…read more

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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 )…read more

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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…read more

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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 angle then you
get total internal reflection ( TIR )
Optical fibres…read more

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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 down the optical
fibre by switching on and off an LED ( Light ­ Emitting
Diode ) or a laser…read more

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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 optics can also be used to send data from one
computer to another or to carry up to ten TV channels
Optical fibres have many advantages over copper cables they
are :
Thinner
Cheaper
Carry…read more

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