Wavelength of an electron revolving in 4th orbit is λ. What is the wavelength of the electron in 2nd orbit?

λ/4  4λ  λ/16  λ/2

λ/2

$ 2\pi r = n\lambda$ $ n\lambda = 2\pi r_0 n^2$ $\Rightarrow \lambda \propto n$ $ \Rightarrow \lambda'= \frac{\lambda}{2}$