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Consider a thin target (10–2m square, 10–3m thickness) of sodium, which produces a photocurrent of 100μA when a light of intensity 100W/m2 (λ = 660nm) falls on it. Find the probability that a photoelectron is produced when a photons strikes a sodium atom. [Take density of Na = 0.97 kg/m3].

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6 × 1026 Na atoms weighs 23 kg.

Volume of target = (10–4 × 10–3) = 10–7m3

Density of sodium = (d) = 0.97 kg/m 

Volume of 6 × 1026 Na atoms=23/0.97 m3=23.7 m 

Volume occupied of 1 Na atom

No. of sodium atoms in the target

Number of photons/s in the beam for 10–4 m2 = n

If P is the probability of emission per atom, per photon, the number of photoelectrons emitted/second

Thus the probability of photoemission by a single photon on a single atom is very much less than 1. (That is why absorption of two photons by an atom is negligible).

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