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 18.0 mA. If the same source is placed 0.6 m away
from the photoelectric cell, then
(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

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 mA`
D. the saturation current will be `18 mA`

Answer 1

Correct Answer - C
Cut off `"voltage"` is independent of intensity and hence remains the same. Since distance becomes `3 times`, so intensity `(I)` becomes `(I)/(9)`. Hence photo current also decreases by this factor i.e., becomes `(18)/(9) = 2 mA`.