Where the classical picture finally breaks down. Light arrives in quanta, atoms keep only certain energies, nuclei decay on their own clock — and every spectrum is a fingerprint. The bench is a black field; whatever is live glows.
The same physics as the benches, written out in full — theory, every solved example, the complete problem bank and a timed chapter test, with the live simulations embedded inline. All five readers are live — the sixth topic, X-rays, lives in the Lab as benches.