Question

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

Answer 1

The correct option is A

Are one quarter as numerous

Number of photo electrons
$\left(N\right)\propto Intensity\propto \frac{1}{d^2}\Rightarrow \frac{N_1}{N_2}={\left(\frac{d_2}{d_1}\right)}^2$
$\Rightarrow \frac{N_1}{N_2}={\left(\frac{100}{50}\right)}^2=\frac{4}{1}\Rightarrow N_2=\frac{N_1}{4}$