Einstein tackled the problem of the photoelectric effect in 1905. Instead of assuming that the electronic oscillators had energies given by Planck's formula (1), Einstein assumed that the radiation itself consisted of packets of energy , which are now called photons. Einstein successfully explained the photoelectric effect using this assumption, and he calculated a value of close to that obtained by Planck.
Two years later, Einstein showed that not only is light quantized, but so are atomic vibrations. Classical physics predicts that the molar heat capacity at constant volume () of a crystal is , where is the molar gas constant. This works well for high temperatures, but for low temperatures actually falls to zero. Einstein was able to explain this result by assuming that the oscillations of atoms about their equilibrium positions are quantized according to , Planck's quantization condition for electronic oscillators. This demonstrated that the energy quantization concept was important even for a system of atoms in a crystal, which should be well-modeled by a system of masses and springs (i.e., by classical mechanics).