Why does the output frequency of a full wave rectifier double that of input frequency?

Answer 1

One cycle of the AC input consists of a positive half-cycle followed be a negative half cycle. The FW rectifier basically reverses the polarity of one of the half-cycles, so there are now two positive pulses per single input cycle, effectively doubling the frequency.

Of course the rectifier may be wired for the opposite polarity, with two negative pulses per input cycle, but the same principle applies.

Answer 2

You can double the output of an input voltage by using voltage multiplier.