A rectifier is an electrical device that converts AC to DC. For example, to rectify the voltage for the resistor R, we insert a diode between the AC supply and R.
A diode turns on when it’s in forward bias, but turns off when it’s in reverse bias. So the diode connects the resistor R to the AC supply during the +ve half cycles, but disconnects it during the −ve half cycles. As a result, VR (the voltage across R) has only the +ve half cycles. This is called half wave rectification.
Since the AC supply connects to R only half the time, the power supplied to R is also halved. Since the rms voltage for the full AC is , the rms voltage for the half-wave rectified AC is . (The rms voltage must be smaller by a factor of in order for the power to be halved). But is there any way to avoid sacrificing half the available power? The answer is in the next section.