You have a triple switch that controls bathroom heat, fan, light combo.
When lights are off inside room, and you flip the heater switch, you see a flash of light inside the switch.
The switch is mounted in electrical box. If flash is inside the box where electric wires are, then that is NOT normal and should be looked after by checking connections and securing wire nuts.http://waterheatertimer.org/How-to-twist-electric-wire.html
If the flash is inside the switch, then that can be normal. But it also might be time to replace switch if switch is old.
What happens inside a switch is that 2 brass connectors come together.
The amp draw from heater is large, so a lot of electricity suddenly passes across the brass connections and produces a flash of light.
If flash is accompanied by a sound, for example crackling or fizzing, and/or you smell electrical burning, then that indicates the switch needs to be replaced.
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