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).