💡 Photoelectric Effect

📖 The Physics

Shine light on a metal and it can knock electrons out. But there's a puzzle: dim blue light works while bright red light does nothing. Einstein explained it by saying light comes in packets called photons, each carrying energy E = hf.

A single photon can eject an electron only if it carries more than the metal's work function W. Below the threshold frequency, even very bright light ejects nothing — this is what proved light is quantised.

Raise the frequency past the threshold and electrons fly off faster (more KE). Turning up the intensity only sends moreelectrons, never faster ones.