Light as a Particle
If you shine light on a metal, it can knock out electrons. But classically, the speed of these electrons should depend on the brightness of the light. Experimentally, it only depends on the color (frequency). Albert Einstein explained this by saying light itself is made of particles called Photons.
\[K_{max} = hf - \Phi\]
where \(\Phi\) is the "work function" (the energy needed to rip the electron out).
Worked Examples
Example 1: Threshold Frequency
If the light frequency is too low (\(hf < \Phi\)), no electrons come out, no matter how bright the light is. This proved that light doesn't deliver energy like a steady stream of water, but like individual "bullets."
The Bridge to Quantum Mechanics
This was the discovery of Wave-Particle Duality. Light is a wave (it interferes), but it is also a particle (it knocks out electrons). This duality is the central mystery of Quantum Mechanics. Einstein won the Nobel Prize for this discovery, not for relativity, because it completely redefined what "matter" and "radiation" are.