posted by james .
determine the wavelength of the line in the emission spectrum produced by an electron transition He+ from ni= 4 to nf= 2.
1/wavelength = RZ^2(1/2^2 - 1/4^2)
R is the Rydberg constant which is 1.09737E7 and Z = 2(don't forget to square it). You can find R more accurately if you go to google.com and type in rydberg constant and click on the wikipedia link.