Energy liberated in the de-excitation of hydrogen atom from 3^ rd level to 1^ st level falls on a photocathode. Later when the same photo-cathode is exposed to a spectrum of some unknown hydrogen like gas, excited to 2^ nd energy level, it is found that the de-Broglie wavelength of the fastest photoelectrons, now ejected has decreased by a factor of 3 . For this new gas, difference of energies of 2^ nd Lyman line and