Question

Three photons coming from excited Atomic hydrogen sample are picked up. their energies are 12.1eV,10.2eV ,1.9eV. these photons must come from

Answer 1

It comes from either two or three atoms.
EXPLANATIN
A hydrogen atom has one electron hence it can only be in one exicted state at a time. Now assuming the photons are all detected at the same time, they would each have to come from a different atom since the H atom can only have one excited state at a given moment in time. In this case, the answer is 3.

Now if these photons are picked up at separate times then it gets more complicated. You have to work out the transitions - what energy level is teh electron moving from and going to for each photon?
You can use the Rydberg formula to do this:

E12 = -13.6*(1/n1^2 - 1/n2^2) eV where n1 > n2 and n1 is the starting energy level and n2 is the final level

THis would involve some trial and error. The 10.1 eV is the transition from n = 1 to n =2 The 12.1 is the n = 3 --> n = 1 transition and the 1.9 eV is the n= 3 ---> n = 2 transition

Now giventhse transitions, only the n =3 --> n = 2 transition levels teh atom in an excited state so the same atom could decay into the ground state by the transition n = 2 --> n =1, But this will occur after the the n= 3 --> n= 2. SO if the photons are received at different times then the it is from either two or three atoms.