Photons 

In the late 1890’s, it was discovered that a beam of light could knock electrons out of a hydrogen atom. The phenomenon became known as the photoelectric effect. You can use Maxwell’s theory to get a rough idea of why a wave of electric and magnetic force might be able to pull electrons out of a surface, but the details all come out wrong. In 1905, in the same year that he developed his theory of relativity, Einstein explained the photoelectric effect by proposing that light consisted of a beam of particles we now call photons.
When a metal surface is struck by a beam of photons, an electron can be knocked out of the surface if it is struck by an individual photon. A simple formula for the energy of the photons led to an accurate explanation of all the experimental results related to the photoelectric effect.
Despite its success in explaining the photoelectric effect, Einstein’s photon picture of light was in conflict not only with Maxwell’s theory, it conflicted with over 100 years of experiments which had conclusively demonstrated that light was a wave. This conflict was not to be resolved in any satisfactory way until the middle 1920s.
The particle nature of light helps but does not solve the problems we have encountered in understanding the behavior of the electron in hydrogen. According to Einstein’s photoelectric formula, the energy of a photon is inversely proportional to its wavelength. The longer wavelength red photons have less energy than the shorter wavelength blue ones. To explain the special colors of light emitted by hydrogen, we have to be able to explain why only photons with very special energies can be emitted.