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

If the work function for the photoelectron emission of a metal is $3.75 \ eV$,then the threshold wavelength of the radiation needed for the ejection of the electron is approximately (in $nm$)

Explain Bohr's atomic model.

When $\alpha$-particles are sent through a thin metal foil,most of them go straight through the foil because (one or more are correct)

The radius of stationary state $(n=2)$ of hydrogen atom is $x \ pm$. The radius of stationary state $(n=3)$ of $He^{+}$ ion (in $pm$) is

Suppose the ionization energy of a hydrogen-like species is $960 \ eV$. The value of the principal quantum number for an energy level of $-60 \ eV$ will be $......$.