AP Physics 2 Unit 7 Essentials

Classical physics treats energy as something that can take any value along a continuum. Modern physics shatters that assumption: light arrives in discrete photon packets (E = hf), and electrons in atoms can only occupy specific, quantized energy levels. This single shift in perspective — from continuous to discrete — is what separates "modern" physics from everything else in the AP Physics 2 course, and it explains phenomena (like the photoelectric effect and atomic spectra) that classical physics simply cannot.

The photoelectric effect shows light — usually understood as a wave — behaving like a stream of particles (photons). The de Broglie relation (λ = h/p) shows the reverse: matter — usually understood as particles — has an associated wavelength and can exhibit wave behavior. Neither "wave" nor "particle" is the complete picture for light or for matter; both are necessary, depending on what experiment you're running.

Einstein's E = mc² isn't just a famous equation — it's the direct explanation for nuclear binding energy, radioactive decay, and the energy release in both fission and fusion. A small, measurable "missing mass" (the mass defect) in any bound nucleus corresponds to an enormous amount of binding energy, because even a tiny mass converts to a huge amount of energy when multiplied by c². This is why nuclear reactions release so much more energy than chemical reactions, which only rearrange electron configurations rather than converting mass into energy.