Physics

The key is understanding how voltage and power distribute differently in series versus parallel circuits. In series, the battery's voltage splits between the two bulbs, so each gets only half the voltage—and since power depends on $V^2/R$, each bulb receives much less power and glows dimly. In parallel, both bulbs experience the full battery voltage simultaneously, so each draws maximum power and glows brightly. The total current from the battery is higher in parallel, but that's what the battery supplies; what matters for brightness is the voltage across each individual bulb.

The key is understanding how voltage and power distribute differently in series versus parallel circuits. In series, the battery's voltage splits between the two bulbs, so each gets only half the voltage—and since power depends on $V^2/R$, each bulb receives much less power and glows dimly. In parallel, both bulbs experience the full battery voltage simultaneously, so each draws maximum power and glows brightly. The total current from the battery is higher in parallel, but that's what the battery supplies; what matters for brightness is the voltage across each individual bulb.