Question

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].

Answer 1

6 × 10²⁶ Na atoms weighs 23 kg.

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

Density of sodium = (d) = 0.97 kg/m³  

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

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).