Question

the brightness of light emitted by a bulb is observed to decrease when an electric heater is connected in series with that reduce the likely reason for this observation

Answer 1

The power of a heater is much more than that of an electric bulb. When two appliances are connected in series, each of the appliance consumes some voltage and cannot operate at its marked voltage. Hence the effective or total resistance of the circuit increases than the resistance of each appliance.
(For example, if there are two appliances and resistance of 1 appliance is 2 ohm and the other is 5 ohm, the total resistance of the circuit becomes 2+5 = 7ohm, so the appliances cannot operate at their marked voltage).

Thus, the brightness of the light emitted by the bulb decreases because it consumes less power.

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