Q. A caesium photocell, with a steady potential difference of $60V$ across, is illuminated by a bright point source of light $50\ cm$ away. When the same light is placed $1m$ away the photoelectrons emitted from the cell
A) Are one quarter as numerous
B) Are half as numerous
C) Each carry one quarter of their previous momentum
D) Each carry one quarter of their previous energy
Explanation
(a) Number of photo electrons
$(N) \propto Intensity \propto \frac{1}{d^{2}} \Rightarrow \frac{N_{1}}{N_{2}} = \left( \frac{d_{2}}{d_{1}} \right)^{2}$
$\Rightarrow \frac{N_{1}}{N_{2}} = \left( \frac{100}{50} \right)^{2} = \frac{4}{1} \Rightarrow N_{2} = \frac{N_{1}}{4}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.