Q:  What is the wavelength of the photon emitted
      when a hydrogen atoms drops from n=3
      to n=2?

          energy in n=3 state:  -1.51 eV

          energy in n=2 state:  -3.40 eV
          -------------------------------
           difference            1.89 eV


                         12400 eV Angstroms
          wavelength  =  ------------------  =   6563 Angstroms
                              1.89 eV


  Q:  Could you see that photon with your eye?

          Yes, since the human eye can sense light
          from about 4000 to about 8000 Angstroms.
          This photon is in the red.