Modern Physics and Atomic/Nuclear Physics
Modern Physics and Atomic/Nuclear Physics
This domain covers quantization, the photoelectric effect, atomic spectra, wave-particle duality, nuclear decay, half-life, binding energy, and fission/fusion. Many questions ask you to interpret energy-level diagrams or apply conservation laws to nuclear reactions.
Core rules and concepts
- Photon energy E = hf = hc/λ. Photoelectric effect: maximum kinetic energy Kmax = hf - φ, where φ is the work function. No emission below threshold frequency, regardless of intensity.
- Bohr model: electrons occupy discrete levels; photon emission occurs when an electron drops to a lower level, with E = hf.
- de Broglie wavelength λ = h/p. Uncertainty principle: ΔxΔp ≥ ħ/2.
- Nuclear notation: mass number A = protons + neutrons; atomic number Z = protons. Alpha decay reduces A by 4 and Z by 2; beta-minus increases Z by 1.
- Half-life: N = N0(1/2)^(t/T). Binding energy comes from mass defect: E = Δmc².
Worked example
A photon with wavelength 400 nm strikes a metal with work function 2.0 eV. Photon energy: E = 1240 eV·nm / 400 nm = 3.1 eV. Kmax = 3.1 - 2.0 = 1.1 eV. If the wavelength were 700 nm, photon energy would be 1.77 eV, below the work function, so no electrons would be emitted.
Traps that cost marks
Thinking brighter light increases photon energy; confusing work function with kinetic energy; treating half-life as linear; and forgetting that mass number and charge are conserved in nuclear equations. In Bohr transitions, the energy difference-not the level itself-sets the photon energy.