Wave-Particle Duality

The dual nature of light refers to light exhibiting both wave and particle properties. Light particles are called  photons and they have an energy equal to the product of Planck’s constant and frequency.  The photoelectric effect is when a metal atom absorbs a photon and emits an electron. The details of how the photoelectric effect works can only be explained by the particle theory of light. In particular, the fact that there was a threshold frequency below which no electrons would be ejected and the fact that the kinetic energy of the emitted electrons was dependent on the frequency of the photon were only explainable by particle theory. 

The Bohr Model of the atom explains the atomic spectrum of hydrogen using fixed electron energy levels. This model would be made more accurate later.