Given that the electron energy in hydrogen atom
En = (-2.18 × 10-18)/n2J
Energy required for ionization from n = 2 is given by
∆E = E∞ – E1
= [0- [(2.18 × 10-18)/ 4] J
= 0.545 × 1018 J
∆E = 5.45 × 10-19 J
Now, λ = he/∆E
= (6.626 × 10-34) (3 × 108)/ 5.45 × 10-19
= 3647 × 10-10 m
= 3647 Å