The energy of an electron in the first Bohr orbit of $H$-atom is $-13.6 \ eV$. The possible energy value of an electron in the first excited state of $Li^{2+}$ is (in $eV$)

The radius of the $2^{nd}$ orbit of $B^{4+}$ ion is:

Bohr's model can explain:

The work function for a caesium atom is $1.9 \, eV$. Calculate $(a)$ the threshold wavelength and $(b)$ the threshold frequency of the radiation. If the caesium element is irradiated with a wavelength of $500 \, nm$,calculate the kinetic energy and the velocity of the ejected photoelectron.

First and second excitation potentials of hydrogen atom (in $eV$) would be respectively

The magnitude of kinetic energy $(K.E.)$ of an electron in a Bohr orbit is equal to which of the following?