An electric bulb is rated as 220 V and 80 W. When it is operated on 110 V, the power rating would be :

The power consumed by an electrical device is calculated using the formula P = V²/R, where P is power, V is voltage, and R is the resistance. The resistance of the bulb remains constant regardless of the operating voltage.

• Step 1: From the initial rating (220 V, 80 W), the resistance is R = V²/P = (220)² / 80.
• Step 2: When the bulb is operated at a new voltage (V' = 110 V), the new power (P') is P' = (V')²/R.
• Step 3: Substituting the value of R, we get P' = (110)² / [(220)² / 80] = (110/220)² × 80.

Since 110 V is half of 220 V, the ratio is 1/2. Squaring this ratio gives 1/4. Therefore, the new power is 1/4 × 80 W = 20 W. Thus, Option D is correct.