This section covers the modern physics that revolutionised our understanding of matter: the quantum nature of light (photoelectric effect), energy levels in atoms, and nuclear physics.
Nucleus: protons (Z) + neutrons (N), nucleon number A = Z + N. Nuclear reactions: conservation of charge (Z) and nucleon number (A). Mass defect: mass of nucleus < sum of individual nucleons. Binding energy = mass defect × c² (Einstein). Binding energy per nucleon: higher = more stable. Fission: heavy nucleus splits (Fe/Ni have highest BE/nucleon). Fusion: light nuclei combine. Both release energy by moving towards Fe peak.
Alpha (α, ⁴₂He): most ionising, least penetrating (paper). Beta-minus (β⁻, ₋₁⁰e): neutron → proton + electron + antineutrino. Gamma (γ): EM radiation after α/β decay, most penetrating. Decay law: N = N₀e^(-λt). Activity A = λN. Half-life t₁/₂ = ln2/λ. Background radiation: cosmic rays, rocks, medical, food. Inverse square law for gamma intensity.
Photoelectric effect: light ejects electrons from metal surface. Observations: (1) below threshold frequency f₀ → no emission; (2) increasing intensity → more electrons, same KE; (3) KE depends on frequency. Einstein: E = hf (photon energy). hf = φ + KE_max → KE_max = hf - φ (φ = work function). Proves particle nature of light. Wave-particle duality: light shows wave (interference, diffraction) and particle (photoelectric) behaviour. Electrons show wave behaviour: de Broglie wavelength λ = h/p = h/mv. Energy levels: electrons occupy discrete energy states. Photon emitted when electron drops: E = hf = E₁ - E₂. Line spectra as evidence.
Classical wave theory predicts: (1) any frequency of light should eventually provide enough energy to eject electrons if given enough time/intensity; (2) brighter light should give electrons more KE. But experimentally: (1) below a threshold frequency, no electrons are emitted regardless of intensity or time; (2) the maximum KE depends only on frequency, not intensity. These observations can only be explained if light comes in discrete packets (photons) each with energy E = hf. One photon interacts with one electron. If hf < φ (work function), no single photon has enough energy, and photons do not combine their energies.
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