Q. 303.8( 5 Votes )

# Light of intensit

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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