collision helps transfer energy
Consider Hydrogen and singly ionised helium mixture in a container. H is present in its 2nd excided state and He+ in 1st excited state.. H and He collides such that H atom gets de-excited and the energy released is completely absorbed by He+ion to re-excite. Find the number of spectral lines observed when the reexcited He+ion gets de-excite to ground state such that there is no spectral line in visible region..!!