Christiaan Huygens (1629–1695) was a Dutch polymath who seemed to do everything at once: he built better telescopes, discovered Saturn's rings and its moon Titan, invented the pendulum clock, and argued that light travels as a wave — all while trading letters with half the great minds of Europe. If there was a hard problem going in the 1600s, Huygens was probably already halfway through it.
While Isaac Newton insisted light was a stream of tiny particles, Huygens said no — light
is a wave, and every point on a wavefront acts as a source of new little wavelets that add
up to the next wavefront. That elegant picture, "Huygens' principle," neatly explained
reflection and refraction. The wave-versus-particle argument he started ran for centuries,
and only ended in the strange truce of
Huygens was obsessed with keeping accurate time — mostly to help sailors find their longitude at sea. His pendulum clock was the most precise timekeeper the world had seen, and he spotted something eerie: two of his clocks hung on the same beam would slowly fall into perfect opposite swing. He'd discovered synchronisation, a phenomenon physicists and engineers are still fascinated by today, from fireflies flashing in unison to power grids staying in step.