Quantum Theory
The photon was the basic unit of energy for the light. The energy in the wave depended on the field strength squared.
[Recall energy in capacitor, Energy = (1/2)CV2, where for parallel plates, Efield = V/d and
Energy = (1/2)*(K?oA/d)*(Efieldd)2
= K?oEfield2 * Vol, or Energy ??Efield2.]