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

The radius of the first Bohr orbit of the hydrogen atom is the same as that of the orbit $(n)$ of a hydrogen-like species $X$. The values of $(n)$ and $X$ respectively are:

The four lowest energy levels of the $H$ atom are shown in the figure. The number of emission lines will be ...... .

For the $H$ atom,which of the following is the correct order of energy?

The ratio of the area enclosed by the second orbit to the first orbit is ......

Ejection of the photoelectron from metal in the photoelectric effect experiment can be stopped by applying $0.5\, V$ when the radiation of $250\, nm$ is used. The work function of the metal is ................ $eV$