Subject: why the observed line spectrum provides evidence for discrete levels

sample of hydrogen gas at very low pressure is subjected to high voltage, 
is emitted and than passed through a prism and adiscrete, specific 
wavelengths which are constant for hydrogen is seen. Why does the line 
spectrum provide evidence for discrete energy levels in the atom? Where 
does photonscome in?
                                                                     

Re: why the observed line spectrum provides evidence for discrete levels