Find wavelength of `1^(st)` line of Paschen series of hydrogen spectrum.

Explanation

In paschen series : `n_(1)=3`
for `1^(st)` line `n_(2)=4`
`(1)/(lambda)=R_(H)[(1)/(3^(2))-(1)/(4^(2))]`
`implies lambda=1.87mum`