In a photoelectric effect, the number of photo electrons emitted per second is directly proportional to
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → intensity of the incident radiation
In the photoelectric effect, the intensity of the light fixes the number of photons striking the metal each second, and each photon ejects at most one electron.
Option (1) frequency of the incident radiation — Incorrect. The frequency decides the maximum kinetic energy of the electrons and whether emission happens at all (threshold frequency), not how many are emitted.
Option (2) wavelength of the incident radiation — Incorrect. The wavelength is linked to the frequency (\( \lambda = c/\nu \)) and so also fixes only the electron energy.
Option (3) intensity of the incident radiation — Correct. For a frequency above the threshold, the photocurrent (number of electrons per second) is directly proportional to the intensity.
Option (4) speed of the incident radiation — Incorrect. All electromagnetic radiation in a given medium travels at the same speed; it is not a variable that changes the emission rate.
Note: A similar question was also asked in CUET (UG) 2022 on 18-08-2022 (Slot 2).
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).”
Einstein’s Photoelectric Equation: Energy Quantum of Radiation – “… The greater the number of energy quanta available, the greater is the number of electrons absorbing the energy quanta and greater, therefore, is the number of electrons coming out of the metal (for ν > ν0).”