In crystals, the electronic energy spectrum has a band structure . Near the minimum of an isotropic energy band, electron energy can be expanded in powers of  as  where  is the electron effective mass. It can be shown that it satisfies the equation
Here the sum runs over all bands with . Therefore, the ratio  of the free electron mass  to its effective mass  in a crystal can be considered as the oscillator strength for the transition of an electron from the quantum state at the bottom of the  band into the same state.