Modern Physics 2 Question 24
23. 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 $18.0 mA$. If the same source is placed $0.6 m$ away from the photoelectric cell, then
$(1992,2 M)$
(a) the stopping potential will be $0.2 V$
(b) the stopping potential will be $0.6 V$
(c) the saturation current will be $6.0 mA$
(d) the saturation current will be $2.0 mA$
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Solution:
- (b) Stopping potential depends on two factors - one the energy of incident light and the other the work function of the metal. By increasing the distance of source from the cell, neither of the two change. Therefore, stopping potential remains the same.
(d) Saturation current is directly proportional to the intensity of light incident on cell and for a point source, intensity
$$ I \propto 1 / r^{2} $$
When distance is increased from $0.2 m$ to $0.6 m$ (three times), the intensity and hence the saturation current will decrease 9 times, i.e. the saturation current will be reduced to $2.0 mA$.
