A point source causing photoelectric emission from a metallic plate is moved away from the plate. The variation of photoelectric current with distance from the source is correctly represented by the graph __________.
Fill in the blank with the correct answer from the options given below.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3)
The photoelectric current is proportional to the intensity of the light falling on the plate, since the number of photoelectrons emitted per second is proportional to the number of photons arriving per second.
For a point source of power \( P \), the light spreads over a sphere, so the intensity at distance \( d \) is
\( \displaystyle I_{\mathrm{light}} = \frac{P}{4\pi d^{2}} \;\Rightarrow\; i \propto \frac{1}{d^{2}} \)
So as the source moves away, the current falls, steeply at first and then more slowly, approaching zero but never reaching it. This is the curve of graph (3).
NCERT (2026-27):
Effect of Intensity of Light on Photocurrent – “… It is found that the photocurrent increases linearly with intensity of incident light as shown graphically in Fig. 11.2. The photocurrent is directly proportional to the number of photoelectrons emitted per second.”