Huygens’ wave theory of light could not explain the phenomenon of:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → photoelectric effect
The wave theory treats light as a wave in which energy is spread continuously over the wavefront. It explains every effect that depends on superposition and wavefronts, but the photoelectric effect depends on light being absorbed in small packets (quanta) of energy, which the wave picture does not contain. Its predictions about intensity, threshold frequency and time lag all disagree with the observations.
Why the other options are incorrect
Option (1) interference — Incorrect. Interference follows from the superposition of waves; Young’s experiment is explained by the wave theory.
Option (2) diffraction — Incorrect. Bending of light at an obstacle or a slit is explained by secondary wavelets.
Option (3) rectilinear propagation of light — Incorrect. The wavelength of light is very small, so over ordinary distances the wavefronts are carried forward and light appears to travel in straight lines; the wave theory accounts for this.
Note: A similar question was also asked in CUET (UG) 2026 on 19-05-2026 (Slot 1), and in CUET (UG) 2025 on 16-05-2025 (Slot 1).
NCERT (2026-27):
Photoelectric Effect and Wave Theory of Light – “… These expectations of the wave theory directly contradict observations (i), (ii) and (iii) given at the end of sub-section 11.4.3. … In short, the wave picture is unable to explain the most basic features of photoelectric emission.”