Photo electric effect reveals the:

The correct answer is Option (2) → particle nature of radiation

The features of the photoelectric effect — a threshold frequency, a maximum kinetic energy that depends on frequency but not on intensity, and emission without any time lag — cannot be explained by the wave picture of light.

Einstein explained them by treating radiation as a stream of photons, each carrying energy \( h\nu \) and each giving all of it to a single electron: \( K_{\max} = h\nu - \phi_0 \). The effect therefore reveals the particle nature of radiation. The wave nature is shown instead by interference and diffraction.

Note: A similar question was also asked in CUET (UG) 2023 on 11-06-2023 (Slot 3).

NCERT (2026-27):
Particle Nature of Light: The Photon – “Photoelectric effect thus gave evidence to the strange fact that light in interaction with matter behaved as if it was made of quanta or packets of energy, each of energy hν.”