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Modern Physics
Photoelectric effect, atomic models, nuclear physics
Key Concepts
- Photoelectric effect: light ejects electrons only above threshold frequency (Einstein)
- Bohr model: electrons in fixed orbits; energy quantised as E_n = −13.6/n² eV
- de Broglie: matter has wave nature; λ = h/mv
- Radioactive decay: first-order process, characterised by half-life
- Mass defect × c² = binding energy of nucleus
Important Formulae
| Photon energy | E = hf = hc/λ |
| Photoelectric effect | KE_max = hf − φ (work function φ) |
| Bohr energy levels | E_n = −13.6/n² eV |
| de Broglie wavelength | λ = h/p = h/mv |
| Radioactive decay law | N = N₀ e^(−λt) |
| Half-life | t₁/₂ = 0.693/λ |
Quick Tips
- Threshold frequency f₀ = φ/h; stopping potential V₀ = KE_max/e.
- Hydrogen spectrum: Lyman (UV, n→1), Balmer (visible, n→2), Paschen (IR, n→3).
- Nuclear fission: heavy nucleus splits; fusion: light nuclei combine — both release energy.
Practice Questions
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