In 1907 Einstein makes his first contribution towards expanding the quantum hypothesis: his work on the specific heat of solids at low temperatures. Nernst confirms Einstein's predictions experimentally – for years this is the most important experimental backing for Einstein's thoughts on quantum theory.
In 1913 Bohr applies the quantum theory to the structure of the atoms – and explains the lines in the light spectrum of hydrogen: the spectrum's structure is based on quantum jumps of electrons. Arnold Sommerfeld expands this atom model further to also interpret the fine structure of spectral lines. The result is a semi-classical atomic theory that becomes the springboard for quantum mechanics, which is formulated in the twenties.