. If we take into account the reality that both the nucleus and electron revolve around their common centre of mass

. During electron transition from a higher state, n₂, to a lower state, n₁, we find that the wavelength of the photon emitted is not given by the formula   where R is the Rydberg constant. The correct wavelength, in that case depends on mass of electron (m) and mass of he nucleus (M) and is given by