Question

if one lamp of a group of five identical lamps in series is removed, the voltage across the empty socket becomes equal to the line voltage, rath than remaining one-fifth of it Why?

Answer 1

In a series circuit, the current that flows through each of the components is the same, and the voltage across the circuit is the sum of the individual voltage drops across each component.

Therefore when a lamp is removed, an open in the circuit will be created. Then no more current is flowing, none of the other lamps will have a voltage drop. Therefore the voltage on the high potential side of the socket will be the input voltage and the low potential side will be at ground.