NCERT Physics Class 12 Exemplar Ch 11 Dual Nature of Radiation and Matter Part 3

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Long Answer

25 Consider a thin target ( square, thickness) of sodium, which produces a photocurrent of when a light of intensity falls on it. Find the probability that a photoelectron is produced when a photons strikes a sodium atom.


  • Na atoms weigh 23 kg.

  • Volume of target

  • Density of sodium

  • Volume of Na atoms

  • Volume occupied of 1 Na atom

  • No. of sodium atoms in the target

  • Number of photons/s in the beam for

  • Energy per s nhν∴

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


This must equal or

  • 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).

26 Consider an electron in front of metallic surface at a distance d (treated as an infinite plane surface). Assume the force of attraction by the plate is given as

Calculate work in taking the charge to an infinite distance from the plate. Taking, find the work done in electron volts. .


Work done by an external agency

With, energy