Modern Physics
Modern Physics: Photons, Models, and Nuclear Change
Modern physics covers the quantum nature of light, atomic models, and nuclear reactions. The photon model holds that light comes in discrete packets of energy E = hf, so higher-frequency light (like ultraviolet) carries more energy per photon than lower-frequency light (like red). The photoelectric effect demonstrates this: electrons are ejected only if the photon energy exceeds the work function, and increasing intensity increases the number of electrons but not their maximum kinetic energy.
The Bohr model explains atomic spectra: electrons occupy discrete energy levels, and a photon is emitted or absorbed when an electron drops or rises between levels, with energy equal to the level difference. Emission spectra appear as bright lines; absorption spectra as dark lines at the same wavelengths.
Wave-particle duality says matter also has a wavelength (de Broglie), inversely proportional to momentum-larger mass or speed means shorter wavelength. The standard model describes matter in terms of quarks and leptons.
Nuclear reactions conserve mass number and charge. Alpha decay emits a helium nucleus (mass 4, charge +2), beta decay converts a neutron to a proton with an emitted electron, and gamma emission releases energy without changing mass or charge. Mass defect appears as released energy via E = mc².
Worked example: A photon of frequency 6.0 × 10¹⁴ Hz has energy E = hf = (6.63 × 10⁻³⁴)(6.0 × 10¹⁴) ≈ 4.0 × 10⁻¹⁹ J. If the work function is lower than this, electrons are emitted with kinetic energy equal to the difference.
Traps: thinking brighter light raises electron kinetic energy in the photoelectric effect (it raises count); confusing emission and absorption lines; and forgetting to balance mass and charge in nuclear equations.