If the electron of a hydrogen atom is in the third orbit,it represents its ...... excited state.

The wavelength of the spectral line obtained in the spectrum of $Li^{2+}$ ion, when the transition takes place between two levels whose sum is $4$ and difference is $2$, is

The electron in the hydrogen atom undergoes a transition from higher orbitals to an orbital of radius $211.6 \text{ pm}$. This transition is associated with:

Calculate the energy of $1 \ mol$ of photons having a frequency of $4.0 \times 10^{14} \ Hz$.

The approximate ratio of the speed of light in vacuum to that of an electron in the first Bohr orbit of hydrogen atom is (in $: 1$)

Explain the phenomenon of black body radiation.