A 100   W electric bulb produces electromagnetic radiation with electric field amplitude of 2   V   m - 1 at a distance of 10   m . Assuming it as a point source, estimate the efficiency of the bulb.

Step-by-step solution

Intensity = power × η A           . . . 1 We know, 1 2 I = 1 2 ε 0 E 2 c I = 8 . 854 × 10 - 12 2 2 3 × 10 8 I = 0 . 0106   w   m - 2                 . . . 2 On substituting 2 in 1 , 0 . 0106 = 100 × η 4 × π × 10 2 ⇒   η = 0 . 133 = 13 . 3 %