The experimental set-up used for the photoelectric effect is shown. It consists of an evacuated glass/quartz tube having a thin photosensitive plate C and another metal plate A. Light from the source S of sufficiently short wavelength passes through the quartz window and falls on the photosensitive plate C (emitter). The electrons emitted by the plate C are collected by the plate A (collector) by the electric field created by the battery. The battery maintains the potential difference between the plates C and A, which can be varied. Thus, the plate A can be maintained at a desired positive or negative potential with respect to the emitter C. When the collector plate A is positive with respect to the emitter plate C, the electrons are attracted to it. The emission of electrons causes a flow of electric current in the circuit, called the photoelectric current.
What is the effect of the intensity of the incident light on the photocurrent?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → It is directly proportional to the intensity.
Option (1) It is inversely proportional to the intensity. — Incorrect. More intense light brings more photons per second. So more electrons are emitted, and the current rises.
Option (2) It is directly proportional to the intensity. — Correct. With the frequency (above the threshold) and the collector potential kept fixed, the intensity sets the number of photons falling per second. One electron absorbs one photon. So the number of photoelectrons emitted per second, and with it the photocurrent, is proportional to the intensity.
Option (4) It does not depend on the intensity. — Incorrect. It is the stopping potential (the maximum kinetic energy) that does not depend on the intensity. The number of electrons emitted per second, and so the current, follows the intensity.
NCERT (2026-27):
Effect of Intensity of Light on Photocurrent – “… Keeping the frequency of the incident radiation and the potential fixed, the intensity of light is varied and the resulting photoelectric current is measured each time. … 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.”