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 \u2013 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 \u2013 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.