Well, all of this we owe it to Bohr who analyzed the atomic emission spectrum of hydrogen and he could probe matematically that it was a result of movement of e- from an especific energy level to a lower one. The understanding of levels of energy took to the development of the atomic theory
the atomic emission spectrum (Bhor analyzed) of hydogren and showed mathematically that the result of movement making a transition from a high energy state to a lower energy.
