The correct option is (C) 128.2 C.
Explanation:
power of beam = 10–3 W and r = radius = 10–3 m
∴ amount of energy incident in unit time = 10–3 J
∴ E = {(nhc) / λ} = 10–3 hence n = {(10–3 λ) / hc}
∴ n = [{10–3 × 5000 × 10–10} / {6.625 × 10–34 × 3 × 108}]---(λ = 5000Å)
= 251.57 × 1013
n' = no of photons per unit area in one second
= [{251.57 × 1013} / {π(10–3)2}] = 80.11 × 1019
each photon emits 1 electron.
∴ amount of charge emitted per unit area in unit times is ɸ and ɸ = n' e
= 80.11 × 1019 × 1.6 × 10–19
= 128.18 C