Which phenomenon established the wave nature of light?

The correct answer is Option (1) → Diffraction

Option (1) Diffraction — Correct. At a narrow slit or a sharp edge light bends and spreads into the region of geometrical shadow. Along with interference, this spreading is explained in a natural way by the wave picture of light, and it established the wave nature of light.

Option (2) Refraction — Incorrect. The corpuscular model of Descartes and Newton also explained the laws of refraction (Snell’s law). So the bending of light at an interface does not by itself decide between particles and waves.

Option (3) Reflection — Incorrect. The laws of reflection follow from the corpuscular model as well as from the wave model, so reflection cannot establish the wave nature of light.

NCERT (2026-27):
Photoelectric Effect and Wave Theory of Light – “The wave nature of light was well established by the end of the nineteenth century. The phenomena of interference, diffraction and polarisation were explained in a natural and satisfactory way by the wave picture of light.”