Key Idea Number of spectral lines abtained due to transition of electron from nth orbit to lower orbit is N=2n(n−1) and for maximum wavelength the difference between the orbits of the series should be minimum. Number of spectral lines N=2n(n−1) ⇒ or or or 2n(n−1)n2−n−12(n−4)(n+3)n=6=0=0=4 Now as the first line of the series has the maximum wavelength, therefore electron jumps from the 4th orbit to the third orbit.