Question
A 5W pointsource emits monochromatic light of wavelength 5000 Å. How many photons per second strike a unit area placed 5 m away from the source and illuminated by it? What should be the work function of the metal from whose surface this light can liberate photoelectrons?



None of these

medium
Solution
The energy of an emitted photon is
.
The power of the source is 5 W, that is, the rate of emission of light is 5 Js^{–1}. Since the energy of one lightphoton is , the number of photons emitted per second from the source is
.
The number of photons striking per second per unit area placed 5 m (= R, say) away from the source is
.
Now,
Work function of the metal = energy of incident photon
= 2.475 eV.
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