In a typical set-up of photo electric effect, the original source of blue light is replaced with a source of red light of testy. Which of the following phenomenon is observed?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Stopping potential decreases
By Einstein's photoelectric equation, the maximum kinetic energy of the photoelectrons, and so the stopping potential, depends on the frequency of the light.
\( \displaystyle e V_0 = h\nu - \phi_0 \)
Red light has a lower frequency than blue light, so \( h\nu - \phi_0 \) is smaller and the stopping potential decreases.
Option (1) Photoelectric current increases — Incorrect. The photoelectric current depends on the intensity of the light, not on its colour (as long as the frequency is above the threshold).
Option (2) Photoelectric current decreases — Incorrect. Same reason: changing the colour alone does not decide the photoelectric current.
Option (3) Stopping potential increases — Incorrect. A lower frequency gives a smaller maximum kinetic energy, so the stopping potential cannot increase.
Option (4) Stopping potential decreases — Correct. Lower \( \nu \) gives smaller \( eV_0 = h\nu - \phi_0 \).
NCERT (2026-27):
Effect of Frequency of Incident Radiation on Stopping Potential – “… (i) For a given photosensitive material and frequency of incident radiation (above the threshold frequency), the photoelectric current is directly proportional to the intensity of incident light (Fig. 11.2).”
Effect of Frequency of Incident Radiation on Stopping Potential – “… Above the threshold frequency, the stopping potential or equivalently the maximum kinetic energy of the emitted photoelectrons increases linearly with the frequency of the incident radiation, but is independent of its intensity (Fig. 11.5).”