The quanta first appeared in modern physics when investigating the nature of thermal radiation, or blackbody radiation. In a sense, it was the beginning of QM. Yet, even though I've read a lot about both BB radiation and quantum theory, I can't really tell you exactly how BB radiation fit into QMs description of radiation. If you read any basic QM text today you'd probably first read about Planck and how he solved the problem with BB radiation by introducing the quanta. You'd probably also read about the Bohr model and schrödinger waves and how photons are emitted when electrons shift energy levels. What I've noticed about these texts though, is that they never make any connecting between these two types of radiation. It's as though they were two separate phenomena. Many texts also give you the impression that blackbody radiation is something unreal, not present in nature. They don't clarify that it's the blackbody that's ideal and unnatural, not the radiation. The fact that BB radiation is emitted by all matter is always left out. I think this is intentional though, not to confuse the reader. Because, to be honest, I'd be confused when first reading how all matter emit continous spectra of radiation, and then reading about atoms and how all matter emit discrete spectra of radiation. Please enlighten me - how does thermal radiation, a phenomena so abundant in nature, fit into QMs desciption of radiation ?