The electron in a hydrogen atom undergoes a transition from higher orbits to an orbit of radius $476.1 \text{ pm}$. This transition corresponds to which of the following series?

In which orbit of the Bohr model of the hydrogen atom is the speed of the electron maximum?

The angular momentum of an electron in an $H$ atom in a particular $n$ state is $\frac{h}{\pi}$. What is the energy in $J$ required to excite the electron from this particular $n$ state to $(n+1)$ state? $(x = 2.18 \times 10^{-18} \ J)$

Two electrons are revolving around a nucleus at distances $r$ and $4r$. The ratio of their time periods is

What is the radius of the fourth orbit of a hydrogen atom (in $pm$)?

The frequency and energy of a photon with a wavelength of $4000 \ \mathring{A}$ are respectively.......