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


                          1240 eV nm
          wavelength  =  -----------  =   656 nm
                           1.89 eV


  Q:  Could you see that photon with your eye?

          Yes, since the human eye can sense light
          from about 400 nm to about 800 nm.
          This photon is in the red.