1. The document discusses the photoelectric effect and how it contradicted classical physics predictions but aligned with Einstein's explanation using quantum theory. 2. Einstein proposed that light behaves as discrete packets of energy called photons, and that photons can eject electrons from a metal surface if they have sufficient energy to overcome the metal's work function. 3. Experiments validated Einstein's explanation by showing that photoelectrons are ejected instantly dependent on photon frequency, not intensity, and with a range of kinetic energies.