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)
The intensity of the incident light is doubled, but its frequency remains slightly less than the threshold frequency. How will it affect the photoelectric emission?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → There is no photoelectric emission
An electron is emitted only when a single photon gives it enough energy. For this, the frequency must be above the threshold frequency of the metal. The intensity sets only the number of photons. It does not change the energy of each photon.
Option (1) It increases — Incorrect. Doubling the intensity doubles the number of photons, but each photon still has a frequency below the threshold, so no electron is freed.
Option (2) It decreases — Incorrect. A higher intensity never reduces the emission. Below the threshold there is no emission to reduce.
Option (3) It remains constant — Incorrect. No photoelectrons come out at either intensity, so there is no steady flow of photoelectrons that could stay the same.
Option (4) There is no photoelectric emission — Correct. Below the threshold frequency no photoelectrons are emitted, however intense the light is, so doubling the intensity changes nothing.
NCERT (2026-27):
Effect of Frequency of Incident Radiation on Stopping Potential – “(iii) For a given photosensitive material, there exists a certain minimum cut-off frequency of the incident radiation, called the threshold frequency, below which no emission of photoelectrons takes place, no matter how intense the incident light is.”