falls
on a sodium photo-cell of surface area 2 cm2. Assuming that the top
5 layers of sodium absorb the incident energy, estimate time required for photoelectric
emission in the wave-picture of radiation. The work function for the metal is given
to be about 2eV. What is the implication of your answer?
Answer:
.
Taking the approximate
radius of an atom as 10-10 m, the effective area of sodium atom is
If
there is one free electron per atom, then number of electrons in five layers
Now Incident
power P = IA
For absorption
of the incident power equally by all electrons, energy absorbed per electron
per second is 
Time required for photo
electric emission to take place 
The answer obtained
implies that the time of emission of electron is very large and is not in
agreement with the observed time of emission, which is approximately 
s.
Thus wave-picture of
radiation is not applicable for photo-electric emission.
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