White radiations are x-rays that have energy ranging between 5-10 eV.When white radiation is passed through a sample of hydrogen gas at room temperature, absorption lines are observed only in the Lyman series.At room temperature, almost all the atoms are in ground state.The minimum energy required for absorption is 10.2eV (for a transition from n=1 to n=2 ). The white radiation has photon radiations that have an energy of around 10.2eV. So, they are just sufficient to transmit an electron from n=1 to n=2 level. Hence, the absorption lines are observed only in the Lyman series.