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.
Sample Practice Questions
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X-rays are produced when:
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Answer: High-speed electrons hit a metal target
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In Rutherford's experiment, most alpha particles passed through the foil because:
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Answer: Atom is mostly empty space
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de Broglie wavelength: λ = h/p. For electron accelerated through V volts:
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Answer: λ = h/√(2meV)
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Binding energy per nucleon is maximum for:
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Answer: Iron (Fe-56)
Practice Questions
Practice 20 randomly selected NEET questions on Modern Physics. Answers are revealed after each question.
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What are the key concepts in Modern Physics for NEET?
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.
Which formulae are important for Modern Physics in NEET?
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.
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