KEAM 2011 Physics and Chemistry Paper

According to Rydberg's formula
‌

1

λ

=R(‌

1

n12

−‌

1

n22

)Z2
For He+ ion; Z=2,n1=2 and n2=4
∴‌‌‌

1

λ

=R(‌

1

22

−‌

1

42

)×22
=R×‌

3

16

×4
‌

1

λ

=‌

3R

4

or ‌‌λ=‌

4

3R

For getting the same value of wavelength in hydrogen atomic spectrum, the values of n1 and n2 must be 1 and 2 respectively.
∴‌‌‌

1

λ

=R(‌

1

12

−‌

1

22

)×12
‌

1

λ

=‌

3R

4

or λ=‌

4

3R