BRIGHTNESS OF A BULB IS  directly  PROPORTIONAL TO voltage  AND inversely PROPORTIONAL TO its resistance.

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Brightness of a bulb is proportional to the electrical power P, dissipated by the filament in the bulb, in the form of heat/light/radiation. 

P = V² /  R 

Brightness is directly proportional to the square of voltage applied across the bulb.      It is inversely proportional to the resistance of the bulb.

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