When hydrogen atom is excited, it returns to its normal unexcited (or ground state) state by emitting the energy it had absorbed previous. This energy is given out by the atom in the form of radiations of different wavelengths as the electron jumps down   

 from a higher to a lower orbit. Transition from dissimilar orbits cause dissimilar wavelengths, these constitute spectral series which are characteristic of the atom emitting them. When observed through a spectroscope, these radiations are imaged as sharp and straight vertical lines of a one color.