Wave-Particle Duality of Electromagnetic Radiation

By the start of the 20th century, physicists faced a genuine dilemma about the nature of light. Phenomena such as diffraction (light bending around obstacles/through slits) and interference (light waves reinforcing or cancelling each other) could only be explained by treating light as a wave. But other phenomena — black-body radiation (the characteristic glow of a heated object) and the photoelectric effect (electrons being ejected from a metal surface when light shines on it) — could not be explained by wave behaviour at all, and were only explained by treating light as a stream of discrete particles. The only way out of this contradiction was to accept that electromagnetic radiation genuinely has a dual character: it behaves as a wave while it is propag …