Read the following passage and answer the questions.
A student uses a photocell to study how the stopping potential \(V_0\) depends on the frequency \(\nu\) of the incident light. She uses one source of light and puts a different filter in front of it for each frequency. The source stays at the same distance from the cell in every run. For each frequency, she records the photocurrent while she changes the potential of the collector plate. Figure (a) shows her results for three frequencies \(\nu_1\), \(\nu_2\) and \(\nu_3\). Each curve meets the potential axis at a different negative potential.
She then repeats the work with emitters of two different metals, A and B. For each metal she plots the stopping potential against the frequency and gets a straight line, as in figure (b). The line for metal A starts at the cut-off frequency \(\nu_0\), and the line for metal B starts at \(\nu_0^{\prime}\). In her experiment, \(\nu_0 = 5.0\times10^{14}\ \mathrm{Hz}\) and \(\nu_0^{\prime} = 8.0\times10^{14}\ \mathrm{Hz}\).
(a)
(b)
Why is the saturation current the same for the different frequencies in figure (a)?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → It depends upon the intensity of the incident light.
The saturation current flows when every emitted photoelectron reaches the collector. So it is set by the number of photoelectrons emitted per second. The passage says the light was adjusted to the same intensity at the various frequencies, and this gave the same value of the saturation current.
Option (1) It depends upon the intensity of the incident light. — Correct. The number of photoelectrons emitted per second is proportional to the intensity. With the same intensity for all three frequencies, the same number of electrons is emitted, so the saturation current is the same.
Option (2) It depends upon the frequency of the incident light. — Incorrect. The frequency changes the energy of the photoelectrons and hence the stopping potential. It does not change their number. Figure (a) shows three frequencies with one saturation value.
Option (3) It depends upon the work function of the metal. — Incorrect. The work function fixes the threshold frequency and the maximum kinetic energy. Above the threshold, the intensity sets the number of photoelectrons per second.
NCERT (2026-27):
Effect of Potential on Photoelectric Current – “… We can now repeat this experiment with incident radiation of the same frequency but of higher intensity I2 and I3 (I3 > I2 > I1). We note that the saturation currents are now found to be at higher values. This shows that more electrons are being emitted per second, proportional to the intensity of incident radiation.”