Telecommunications: Communication Using Cables

• sound is converted directly into an electrical signal at the mouthpiece
• which is then sent along a wire to an earpiece
• where it is converted back into sound

• the mouthpiece contains a microphone, which converts sound into electrical energy
• the earpiece contains a loudspeaker, which converts electrical energy into sound energy

a long, thin thread of glass through which light can travel

• smaller in size
• cheaper to make
• lighter
• greater signal capacity
• higher signal quality
• less energy loss per km
• not affected by interference
• not easily tapped

when light is reflected from a plane mirror it is found that the angle of incidence is equal to the angle of reflection

• light signals only travel at 2x10⁸
• as opposed to almost 3x10⁸ for electrical signals in a wire

if the direction of a ray of light is reversed it will follow the same path, but in the opposite direction

• because of total internal reflection
• this is when all the light is reflected inside the glass and none escapes into the air
• total internal reflection occurs when the angle of the incident ray is greater than an angle known as the critical angle
• in this way light can travel great distances through optical fibres without ever leaving the fibre

• electrical signals are converted into pulses of light by laser
• these pulses of light then travel down the optical fibre and at the other end are converted back into electrical signals by a photodiode
• the laser and photodiode are so fast acting that hnudreds of different signals can be transmitted down the same fibre simultaneously