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

Answer

(i) `0.6` volt, (ii) `2.0` mA

Explanation

Number of photons falling/s
`n prop 1/r^(2)` [for point source]
So for new distance `n'=n/9`
`I'_(s)=I_(s)/9=(18 mA)/(9)=2 mA`
Also saturated current `prop n`
`V_(d)` is independent of n.
(i) `V_(S)=0.6 V` (ii) `I_(s)=(18xx(0.2)^(2))/((0.6)^(2))=2 mA`