Assertion (A) : If the filament of a light bulb is not uniform, then its life is shortened. Reason (R) : Resistance of glowing light bulb is less than that of bulb is room temperature.

Assertion (A) is true because a non-uniform filament leads to localized 'hot spots'. In areas where the filament is thinner, the electrical resistance is higher than average [t2][t4]. This causes the thin spot to get hotter than the rest of the filament, accelerating tungsten evaporation in a positive feedback loop until the filament breaks or 'fuses' [t2][t5]. Reason (R) is false because tungsten, the metal used for filaments, has a positive temperature coefficient of resistance [t1]. This means its resistance increases significantly as temperature rises. The cold resistance of a tungsten lamp is typically only about 1/15th of its hot-filament resistance during operation [t2]. For instance, a 100-watt bulb may have a resistance of 144 ohms when glowing but only 9.5 ohms at room temperature [t2]. Therefore, the resistance of a glowing bulb is much higher, not less, than at room temperature.