Bouncing X-rays off electrons proved that light particles carry momentum
In 1923, Arthur Compton fired X-rays at electrons and found the scattered X-rays came out at a longer wavelength — meaning they'd lost energy and momentum in the collision, exactly as if light were made of particles bouncing off matter like billiard balls. It was decisive proof that photons carry momentum, cementing the particle nature of light that Einstein's photoelectric effect had only hinted at, and won Compton the 1927 Nobel Prize in Physics.