Number of photoelectrons emitted per second is proportional to :

The correct answer is Option (1) → Intensity of incident radiation

In the photon picture, each photon can free at most one electron. For a frequency above the threshold, the number of photons falling per second, and so the number of photoelectrons emitted per second (the photocurrent), is proportional to the intensity of the incident radiation.

Option (1) Intensity of incident radiation — Correct. The photocurrent increases in direct proportion to the intensity, for a fixed frequency above the threshold.

Option (2) Frequency of incident radiation — Incorrect. The frequency decides the maximum kinetic energy of the photoelectrons, not how many are emitted per second.

Option (3) Stopping potential — Incorrect. The stopping potential measures the maximum kinetic energy, \( eV_0 = K_{\mathrm{max}} \); it does not depend on the intensity.

Option (4) Wavelength of incident radiation — Incorrect. The wavelength, like the frequency, affects the energy of each photoelectron, not their number.

Note: A similar question was also asked in CUET (UG) 2025 on 14-05-2025 (Slot 2).

NCERT (2026-27):
Effect of Intensity of Light on Photocurrent – “… The photocurrent is directly proportional to the number of photoelectrons emitted per second. This implies that the number of photoelectrons emitted per second is directly proportional to the intensity of incident radiation.”