The Photoelectric Effect: Photons and Planck's Constant
How light knocks electrons out of metal — and why that forced physics to accept the photon
Big Idea: When light shines on a metal, it can eject electrons from the surface. Experiments show this photoelectric effect obeys rules that a wave picture of light cannot explain. Einstein solved it by saying light is delivered in indivisible packets of energy called photons, each carrying an energy E = hf. One photon, one electron. This single equation ties together colour, energy and the strange all-or-nothing behaviour of the effect, and it earned Einstein the 1921 Nobel Prize.
By the end of this topic you will be able to:
- describe the photoelectric effect and the four experimental laws it obeys;
- explain why classical wave theory fails and how the photon idea rescues it;
- use Einstein's photoelectric equation hf = A + Ek(max);
- calculate photon energy, work function, threshold (red) frequency and stopping voltage;
- state the value and units of Planck's constant and use it in problems.