##1/lambda## = ##R_(oo)####((1)/(n_1^(2))-(1)/(n_2^(2)))##

##R_oo## is the Rydberg constant and has the value of

##1.097## x ##10^(7) m^(-1)##

So :

##(1)/(lambda)## = ##1.097## x ##10^(7)## ##((1)/(3^3)-(1)/(6^2))##

##(1)/(lambda)## = ##9.15## x ##10^5m^(-1)##

##lambda## = ##1.096## x ##10^(-6)m##

This is equal to ##1096nm##. The optical window, which our eyes can detect, is between ~##400 – 700nm## so we couldn’t see this spectral line as it is in the near infra – red part of the spectrum.