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A monochromatic light of wavelength λ is incident on an isolated metallic sphere of radius a. The threshold wavelength is λ0 which is larger than λ. Before the emission of photoelectrons stops the number of photoelectrons emitted are (V is the stoping potential) 

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Correct option :  (3)

Explanation : 

The metallic sphere becomes positive charged as the electron are emitted from it (sphere is isolated). Hence the electrons are subjected to an extra force. If the potential of sphere is raised to V, the energy the electrons should possess (minimum) so that photoemission is possible is eV + ω0. The emission of photoeelctron will stop when

The charge on the sphere which will raise the sphere potential to V is Q = (4πɛ0a) V

Hence the number of electrons emitted will be n = Q/e = 4πɛ0aV/e

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