Question

Light from a bathroom bulb gets dimmer for a moment when the geyser is switched on. Why?

Answer 1

When geyser is connected there is a substantial increase in the line current. So the drop due to line resistance increases(line current * line resistance). Because of this available voltage for the bulb comes down(Vline - line drop means a voltage drop along an electric transmission or distribution line due to the impedance of the circuit). So it becomes dimmer. It is not becoming dimmer for a moment but continously it will be dim for sometime.

Or simply we can say in most cases Light from bathroom bulb gets dimmer for a moment when geyser is switched on because load of electicity suddenly increases due to the fact that geyser draws very large start-up current. Since the geyser and the bulb in bathroom are connected in same circuit,so as we switch on the geyser it takes its part of current leaving less current for bulb and after sometimes it gets normal.