Question

Three photons coming from excited atomic-hydrogen sample are picked up.

Answer 1

So what should I do to ans

Answer 2

These energies are due the hydrogen electron dropping down from a higher excited energy level to a lower level .. so they must each be photons from a single atom (electrons don't move between atoms when they change energy levels)

12.1eV .. emitted when an electron drops from level n=3 (1.51 eV) to n=1 (13.60eV)

[excess energy 13.6 - 1.51 = 12.09eV]

10.2 eV .. emitted for a transition between n=2 (3.4eV) and n=1 (13.6eV)

1.9eV .. emitted for a transition between n=3 (1.51eV) and n=2 (3.40eV)