Explanation
It mixes weak signals with carrier signals.
Frequency modulation requires much wider channel (7 to 15 times) as compared to AM.
An antenna is a metallic structure used to radiate or receive EM waves.
Optical fibres are not subjected to electromagnetic interference from outside.
In optical fibre, light travels inside it, due to total internal reflection.
Few advantages of optical fibres are that the number of signals carried by optical fibres is much more than that carrier by the Cu wire or radio waves. Optical fibres are practically free from electromagnetic interference and problem of cross talks whereas ordinary cables and microwave links suffer a lot from it.
Ruby laser is a three level laser. In ruby laser, the stimulated emission is due to transition from metastable state to ground state.
Duplex or full duplex refers to the simultaneous transmission of data in two directions. A mobile phone is a full duplex device because both people can talk at once and hear each at the same times. Walky-talky is a half duplex device because only one person can talk at a time.
Laser action involves all of the following
(i) Amplification of particular frequency
(ii) Population inversion
(iii) Stimulated emission
The frequency of AM channel is 1020 kHz whereas for the FM it is 89.5 M1iz (given). For higher frequencies (MHz), space wave communication is needed. Very tall towers are used as antennas.
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