What transition in the hydrogen spectrum would have the same wavelength as the Balmer transition `n = 4` to `n=2` of ` He^(o+)` spectrum?

Explanation

For `He^+ 1/( lambda) = R_H . Z^2 [1/2^2 - 1/4^2]`
For ` H, 1/(lambda) = R_H [ 1/n_1^2 - 1/n_2^2]`
` :. Z^2 [ 1/2^2 - 1/4^2 ] = [ 1/n_1^2 - 1/n_2^2]` (Since lambda is same )
` :. [ 1/1^2 - 1/2^2 ] = [ 1/n_1^2 - 1/n_2^2]`
`:. n_ 1 = 1 ` and ` n_2 =2`
Thus , ` E_(2-1)` transition for H-atom will have same wavelength as ` E_(4 -2)` transition in `Fe^(+)`.