be the energy of the nth level of H-atom. If all the H-atoms are in the ground state and radiation of frequency (E2-E1)/h falls on it,
(a) it will not be absorbed at all
(b) some of atoms will move to the first excited state.
(c) all atoms will be excited to the n = 2 state.
(d) no atoms will make a transition to the n = 3 state.