Unit 7: Quantum, Atomic, and Nuclear Physics

Executive Summary

At the smallest scales physics becomes quantum: photons, quantized atomic levels, and nuclear transformations.

Photons and the photoelectric effect

Light arrives in quanta of energy $E = hf = \frac{hc}{\lambda}$. The photoelectric effect: electrons eject only above a threshold frequency; intensity controls how many, not their energy — Einstein's Nobel-winning evidence for photons.

Atomic models

Bohr's model quantized orbits; electron transitions emit or absorb photons ($E = E_i - E_f$). Wave-particle duality ($\lambda = h/p$, de Broglie) applies to matter itself. The uncertainty principle limits simultaneous position-momentum precision.

Nuclear physics

Nuclei hold protons via the strong force; mass defect becomes binding energy ($E = mc^2$). Radioactive decay: α (He nuclei), β (electrons), γ (photons), with half-lives governing decay rates. Fusion powers stars; fission powers reactors and the bomb.

Exam traps

Photon energy depends on frequency only — intensity changes the count, not each photon's energy. Higher frequency light does NOT always eject electrons faster (photoelectric threshold). Half-life is statistical — after two half-lives, a quarter remains. Mass and energy interconvert in nuclear reactions.