SSC CGL Physics · Modern Physics
Radioactivity and Quantum Physics
What alpha, beta-minus, beta-plus and gamma decay do to a nucleus, the photoelectric effect and Bohr's energy levels, and the physicists SSC pairs with their discoveries.
Why this matters
Thirteen questions in the bank: six from the 2023 shifts and seven from the 2025 shifts, none from 2024. The 2025 shifts asked mostly about decay, so the decay table is worth learning cold.
Concept 1 of 2: Radioactive decay
Definition
The decays:
- Alpha: emits a helium-4 nucleus; Z falls by 2, A by 4.
- Beta-minus: a neutron becomes a proton; Z rises by 1, A unchanged.
- Beta-plus: a proton becomes a neutron; Z falls by 1, A unchanged.
- Gamma: no change in Z or A; only energy leaves.
Facts:
- Most of Earth's argon is argon-40 from the decay of potassium-40 in rocks (the basis of K-Ar dating).
- Geiger-Nuttall law (1911): alpha emitters with shorter half-lives give more energetic alpha particles.
Practise this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 1 · Modern Physics · Radioactivity and Quantum Physics
Beta-plus lowers Z
Concept 2 of 2: Quantum ideas and the physicists
Definition
Ideas:
- Photoelectric effect: light acts as particles (photons). Below a threshold frequency no electron leaves, however bright the light. Brighter light gives more electrons, not faster ones; their energy depends on frequency.
- Bohr model: E = minus 13.6 / n squared eV, so the gaps between levels shrink as n grows.
Physicists:
- Victor Hess: cosmic rays. C. T. R. Wilson: the cloud chamber (Nobel 1927, with Compton).
- Cornell and Wieman: the first Bose-Einstein condensate (1995).
- Marconi: radio signals over kilometres in the 1890s.
- Bardeen: superconductivity theory. de Broglie: wave nature of matter. Dirac: antimatter (not liquid helium).
Practise this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 2 · Modern Physics · Radioactivity and Quantum Physics
Intensity changes the count, not the energy
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (2)
Radioactive decay4 rows
Watch out for (2)
- Beta-plus lowers Z→ Radioactive decay
- Intensity changes the count, not the energy→ Quantum ideas and the physicists
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