Q:  Note the line labelled "O VIII α" in the spectrum above.
        It represents the n=2 -> n=1 transition of a seven-times-
        ionized oxygen atom.

      What _should_ the wavelength of this transition be?

                                               1 
            E(n)  =   (-13.6 eV) * (8)^2  *  ------
                                              n^2



            E(1)  =  -870 eV

            E(2)  =  -218 eV
                                                
            delta_E  =  652.4 eV    

        
          So the wavelength should be

                               1240 eV*nm 
                lambda   =    ------------  =  1.9  nm   =  19 Angstroms
                                652.4 eV


      What is the wavelength in this spectrum?


                     19 Angstroms