Question

When a monochromatic point source of light is at a distance of 0.2 m from a photoelectric cell, the cut-off voltage and the saturation current are respectively 0.6 V and 18 mA. If the same source is placed 0.6 m away from the photocell, then :

• A. The stopping potential will be 0.2 V
• B. The satuartion current will be 6 mA
• C. The stopping potential will be 0.6 V
• D. The saturation current will be 2 mA

Answer 1

Answer: (C), (D)

The correct options are
C The stopping potential will be 0.6 V
D The saturation current will be 2 mA

Intensity of photons $\uparrow$, no. of electron $\uparrow$

The cut off voltage does not depend on distance between cell and source.

But intensity is inversely proportional to square of it's distance.

$\therefore$ at $d=0.6m,$

Cut off voltage $=0.6V$

but

$I_s=18\times (\frac{0.2}{0.6}{)}^2$

$=2mA$

So, the answers are option (C) & (D).