Answer :

Given:


Intensity of light, I = 10-5 Wm-2


Surface area of photo-cell, A = 2 cm2 = 0.0002 m2


Work function of sodium, Φ0 = 2eV = 2 × 1.6 × 10-19 = 3.2 × 10-19J


By knowing the effective area of each sodium atom we can find the absorption of incident energy,


Effective area of each sodium atom, ANa = 10-20 m2


Number of layers of sodium, n = 5


Number of atoms absorbing radiation, NNa = n × (ANa/ A)


NNa = 5 × (10-20 m2/ 0.0002 m2)


NNa = 1017


So we conclude that 1017 atoms are effectively absorbing radiation.


Energy absorbed per atom, E = I/NNa


E = 10-5Js-1/1017


E = 2 × 10-26 Js-1


Times required for photo electric emission, t is give by,


t = Φ0/E


t = 3.2 × 10-19 J / 2 × 10-26 Js-1


t = 1.6 × 107 s


t = 0.5 years


Hence the time requires to initiate photoelectric emission is 0.5 years which is impractical hence, the wave model stands in disagreement with the experimental results.


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