College Physics 10th Edition by Raymond Serway,Chris Vuille

Exercise 19

A general expression for the energy levels of oneelectron atoms and ions is
Here µ is the reduced mass of the atom, given by µ = m 1 m 2 /( m 1 + m 2 ), where m 1 is the mass of the electron and m 2 is the mass of the nucleus; k e is the Coulomb constant; and q 1 and q 2 are the charges of the electron and the nucleus, respectively. The wavelength for the n = 3 to n = 2 transition of the hydrogen atom is 656.3 nm (visible red light). What are the wavelengths for this same transition in (a) positronium, which consists of an electron and a positron, and (b) singly ionized helium Note: A positron is a positively charged electron.