Light is purely a continuous wave and cannot behave as discrete particles.

By the nineteenth century, the wave theory of light had won major victories. Interference and diffraction were naturally wave phenomena, and Maxwell’s equations later identified light as an electromagnetic wave. The older corpuscular picture seemed unnecessary.

The photoelectric effect exposed a limit. Shining light on a material could eject electrons, but the energies of those electrons depended on the light’s frequency in a way a purely classical continuous-wave picture did not explain well.

In 1905, Einstein proposed that light exchanged energy in discrete packets. The idea was initially controversial because it seemed to revive particle-like light after the wave model had been so successful. Later evidence, including Compton scattering, strengthened the case for quantized electromagnetic radiation.

Quantum theory kept both sets of observations. Light propagates and interferes in ways associated with waves, while interactions can occur in discrete quanta called photons.

This is not a literal story in which a photon alternates between being a tiny billiard ball and a water wave. Classical waves and classical particles are both incomplete analogies. Wave optics remains extremely accurate for many applications, but it is not the whole theory of light.