NCERT Physics Class 12 Exemplar Ch 11 Dual Nature of Radiation and Matter Part 3 (For CBSE, ICSE, IAS, NET, NRA 2022)

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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