Vacuum Diode as Rectifier

What is rectification How can a diode be used as full wave rectifier?

How a semiconductor diode can be used in half wave rectification?

A rectifier is an electronic device which converts alternating current into direct current.

The minimum number of diodes for a full wave rectifier is _____ (write your answer in words)

When $r_p<<<R_L$, The efficiency of a full-wave rectifier is nearly ______ $\%$. (Round off the answer to a single decimal place.)

A voltage $200 \sin 100 \mathrm{\pi t} \mathrm{volt}$ is supplied to a half-wave vacuum rectifier. If $r_p=1 \mathrm{k\Omega } and R_L=10 \mathrm{k\Omega }$. Calculate a.c. power input. (Write the numerical answer in terms of $\mathrm{mW}$.)