Quick questions on Diodes and rectification explained: O-Level Electronics Electronic Components

A diode allows current to pass in one direction (forward) and blocks it in the other (reverse). Its symbol is a triangle pointing towards a bar; conventional current flows in the direction the triangle points. The lead at the triangle end is the anode and the lead at the bar is the cathode, usually marked with a stripe on the body.

A silicon diode does not conduct until the forward voltage reaches about $0.7\ \text{V}$. Below this it stays off; above it, it conducts and holds roughly $0.7\ \text{V}$ across itself. In any calculation you must subtract this $0.7\ \text{V}$ from the supply before working out the current in the rest of the circuit. (A light-emitting diode has a larger forward drop, around $2\ \text{V}$.)

An alternating supply reverses direction every half cycle. Place a single diode in series with the load and it conducts only when its anode is positive, passing the positive halves of each cycle and blocking the negative halves. The result is a series of one-way pulses: the current now flows in one direction only. This is half-wave rectification, the simplest way to turn alternating current into direct current.