While studying the spectrum of a gas cloud in space, an astronomer magnifies a spectral line that results from a transition from a p state to an s state. she finds that the line at 575.050 nm has actually split into three lines, with adjacent lines 4.20 x 10^-2 nm apart, indicating that the gas is in an external magnetic field. (ignore effects due to electron spin.) what is the strength of the external magnetic field?