Q:  What is the wavelength of the photon emitted
      when a hydrogen atoms drops from n=2
      to n=1?


          energy of photon  E  =  10.2 eV



                          1240 eV nm
          wavelength  =  -----------  =   121 nm
                           10.2 eV


  Q:  Could you see that photon with your eye?

          No, since the human eye can sense light
          from about 400 nm to about 800 nm.
          This photon is in the ultraviolet.