Wave–Particle Duality

Classical physics loved clean categories. A particle was a small, localized object with a position and a trajectory. A wave was an extended disturbance that spread through space and showed interference and diffraction. That tidy division worked for centuries — until experiments started showing that nature had no interest in keeping our categories neat.

Light rebelled first. In some experiments it behaved unmistakably like a wave: it diffracted, interfered, and painted patterns that only made sense if light spread out like a ripple. But in others — above all the photoelectric effect — light behaved as if it arrived in localized packets, handing over energy in discrete chunks. Einstein's explanation of those packets, later called photons, didn't erase the wave picture. It stood beside it, and the tension stayed.

Then matter joined the rebellion. Electrons, which everyone expected to behave like particles, produced diffraction patterns too. De Broglie's bold idea — that matter has a wavelength — was confirmed by experiment. Now both sides of the old divide were broken: light sometimes acted like particles, and particles sometimes acted like waves.

If the setup is sensitive to interference, wave-like behavior shows up. If it records localized detection events, particle-like behavior shows up. Neither classical label is the whole story on its own — and the double-slit experiment, coming up next, compresses this entire dilemma into one unforgettable image.