Which of the following statements regarding the photoemission of electrons is correct?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Photoelectric emission is instantaneous with the incidence of light.
Option (1) The kinetic energy of the electrons increases with the intensity of the incident light. — Incorrect. The maximum kinetic energy \( K_{\mathrm{max}}=h\nu-\phi_0 \) depends on the frequency and the metal. A higher intensity only increases the number of photoelectrons.
Option (2) Electrons are emitted when the wavelength of the incident light is above a certain threshold wavelength. — Incorrect. Emission needs \( \nu\gt\nu_0 \), that is, a wavelength below the threshold wavelength \( \lambda_0=c/\nu_0 \). Longer wavelengths give photons too little energy.
Option (3) Photoelectric emission is instantaneous with the incidence of light. — Correct. The emission is the absorption of one photon by one electron. This is an elementary process, so it starts without any apparent time lag (about \( 10^{-9}\,\mathrm{s} \) or less), even in very dim light.
Option (4) Photoelectrons are emitted whenever a gas is irradiated with UV light. — Incorrect. Emission is not automatic for ultraviolet light. It takes place only when the frequency is above the threshold frequency of the material. Ultraviolet light below that threshold frees no electrons.
NCERT (2026-27):
Effect of Frequency of Incident Radiation on Stopping Potential – “(iv) The photoelectric emission is an instantaneous process without any apparent time lag (~10–9 s or less), even when the incident radiation is made exceedingly dim.”