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Φ =2eV

1/ λ =R(1/n1^2 - 1/n2^2)

E =Φ +K.E=hc/ λ

=hcR(1/n1^2 / 1/n2^2)

=Φ + K.E

K.E = eVo

n1=2,n2=n

hcR(1/2^2 - 1/n^2)

=2x1.6x10^-34x1.6x10^-19x0.55

6.62x10^-34x3x10^8x1097x10^7(1/4 - 1/n^2)

=3.2x1.6x0.55x10^-38

1/4 - 1/n^2 = 3.2x1.6x0.55x10^-38 / 6.62x10^-34x3x197x10^15

=2.86x10^-38 / 21.786x10^-19

=1/4 - 1/n^2 = 0.129x10^-19

=1/4 - 1/n^2 = 1.13x10^-20

On solving the above equation we get ,n=2

Hope it helps!!!!