The photoelectric effect — Einstein's Nobel

ELI5: shine light on metal and it kicks out electrons — but only if each individual photon carries enough energy to pay the "exit fee" (the work function W₀). Crank up the brightness of too-red light and nothing happens: it's the photon's frequency, not the intensity, that matters. That single fact shattered the pure-wave picture of light.

Real-world hook: the photoelectric effect runs solar cells, the light meters in cameras, automatic doors, and the photomultipliers in medical scanners.