Question

A photo-cell is illuminated by a source of light, which is placed at a distance $d$ from the cell. If the distance become $d/2$, then number of electrons emitted per second will be :

• A. Remain same
• B. Four times
• C. Two times
• D. One-fourth

Answer 1

The correct option is D One-fourth

Intensity, $I=\frac{E}{At}$

where E is the total energy of source,

A is area of illuminated surface,

t is time

$\therefore I=\frac{E}{4\pi r^{2}t}\frac{I_1}{I_2}=\frac{E}{4\pi r_1^{2}t}\times \frac{4\pi r_2^{2}t}{E}=\frac{r_2^2}{r_1^2}\frac{I_1}{I_2}=\frac{4}{1}\Rightarrow \frac{I_2}{I_1}=\frac{1}{4}$

Since, $\propto$ number of photoelectrons emitted

Therefore, Number of electrons emitted is a quarter of the initial number.