a. A stopping potential of `0.82V` is required to stop the emission of photoelectrons from the surface of a metal by light of eavelength `4000 A`. For light of wavelength `3000 A`, the stopping potential is `1.85V`. Find the value of Planck's constant. [1 electrons volt (eV)`=1.6xx10^(-19)J]`
b. At stopping potential, if the wavelength of the incident light is kept fixed at `4000 A`, but the intensity of light increases two times, will photoelectric current be botained ? Give reasons for your answer.

Explanation

(a) `(hc)/(lambda_(1)) =phi + eV_(s_(1))` …..(i)
`(hc)/(lambda_(2)) =phi + eV_(s_(2))` ….(ii)
(ii) - (i)
`(hc)/(lambda_(2)) - (hc)/(lambda_(1)) =e(V_(s_(2)) - V_(s_(1)))`
`h=(e)/(c) (V_(s_(2)) - V_(s1))lambda_(1) lambda_(2))/(lambda_(1) - lambda_(1))`
`=(1.6xx10^(-19)(1.85-0.82)(4000xx3000)xx10^(-10))/(3xx10^(8)xx(4000-3000))`
`=6.6xx10^(-34` J-sec
(b) At stopping potential, photoelectric current is zero.