With the approach of the twentieth century, the classical wave theory of radiation—a widely accepted theory in physics—began to encounter obstacles.
Topic
Wave theory of radiation
Framework
old / new
Paragraph 1 The very first sentence warns us that we're probably reading an Old/New type passage (along with the fact that Old/New is the most common framework for Science) passages.
An accepted theory is encountering obstacles?
When you suspect an Old/New framework, you try to proactively predict the New, by defining it as the logical opposite of the Old (in the hopes of priming your brain so that it's easier to process the payoff sentences when they arise).
Old New all EM radiation exists some EM radiation in the form of waves isn't in wave form?
as length of wave as length of wave shortens, its energy shortens, its energy increases smoothly doesn't go up like (volume dial) a smooth volume dial?
Any conceivable some energy values energy value could can't occur in occur in nature nature?
Paragraph 2 The transition sentence warns us that we're about to hear the story of the "obstacles" that wave theory started to encounter. Many science passages with an Old/New structure have a section where they're saying: Scientists thought / assumed X, which meant that Y should be true. But then data showed that Y wasn't true!
This happens when we hear that physicists originally predicted they would find more energy at short wavelengths, however they found almost none.
This tension is what sets us up for the New.
Paragraph 3 The second sentence gives us Planck's answer to this conundrum -- "get rid of the Old volume dial / continuous view of energy, start using a New incremental-click / discontinuous view".
We even get a mini Old/New at the end of the paragraph as we learn that "at first" the physics community was deeply reluctant to accept this, since Planck had no physical explanation to back this up.
Paragraph 4 This paragraph opens with a continuator that says Einstein and others came to the rescue by coming up with a different theory of energy that could explain why Planck's incremental-click / discontinuous view could be legit.
The big breakthrough here was the idea of positing particles (photons) that are discrete units and certain wavelengths. (a particle is a quantum of energy, a specific quantity of energy ... which is where the name quantum physics comes from )