Historically, data regarding quantum phenomena has been obtained from two main sources. Firstly, from the study of spectroscopic lines, and, secondly, from scattering experiments. We have already developed theories that account for some aspects of the spectra of hydrogen, and hydrogen-like, atoms. Let us now examine the quantum theory of scattering.
Thumbnail: Collimated homogeneous beam of monoenergetic particles, long wavepacket which is approximately a planewave, but strictly does not extend to infinity in all directions, is incident on a target and subsequently scattered into the detector subtending a solid angle. The detector is assumed to be far away from the scattering center. Image used with permission (Department of Physics Wiki @ Florida State University).