A 35Watt and a 100watt bulbs are joined in series and connected to mains. which will glow brighter? what will happen when the bulbs are connected in parallel?

To understand this, you must first understand that the filament in a 40-watt bulb is thinner and therefore of higher resistance than the filament of a 100-watt bulb. It is the higher resistance of the 40-watt bulb that keeps the current to only 40/100 the current in a 100-watt bulb when both are properly connected in parallel. Then more current flows in the 100-watt bulb and it glows brighter.

But when connected in series, the current is less but is the same in each. The same amount of current "squeezing" through the finer filament of the 40-watt bulb heats it more and makes it glow brighter than the lower-resistance 100-watt bulb.

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regards

Ramesh