A monochromatic light of wavelength `lambda` is incident on an isolated metalic sphere of radius `a`. The threshold wavelength is `lambda_(0)` which is larger then `lambda`. Find the number of photoelectrons emitted before the emission of photo electrons stops.

Explanation

As the metallic sphere is isolated, it becomes positively charged when electrons are ejected from it.There is an extra attractive force on the photoelectrons. If the potential of the sphere is raised to `V`,the electron should have minimum energy `W+eV` to be able to comeout. Thus, emission of photoelectons will stop when
`(hc)/lambda=W+eV=(hc)/lambda+eV` or `V=(hc)/e(1/lambda-1/lambda_(0))`.
The charge on the sphere needed to take its potential to `V` is `Q=(4piepsilon_(0)a)V`
The number of electrons emitted is, therefore,
`n=Q/e=(4piepsilon_(0)aV)/e=(4piepsilon_(0)ahc)/e^(2)(1/lambda-1/lambda_(0))`