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Modern physics
Physics
Useful for: OSSC RHT — Regular High School Teacher, OSEPA Upper Primary — Science, OSSC Science, OSSSC Govt TGT Science — PCM, SSB Lecturer (Degree Colleges), SSB TGT PCM
Overview
Modern physics explains phenomena at atomic and nuclear scales, such as the photoelectric effect, atomic spectra, radioactivity and nuclear energy.
Key points
- Einstein explained the photoelectric effect using photons and received the Nobel Prize in 1921.
- Rutherford’s gold-foil experiment showed a small dense nucleus; Bohr proposed fixed energy levels.
- X-rays were discovered by Wilhelm Röntgen; radioactivity by Henri Becquerel.
- Alpha particles are helium nuclei; beta particles are electrons; gamma rays are electromagnetic waves.
- Half-life is the time for half the radioactive nuclei to decay.
- Mass–energy equivalence: E = mc² explains nuclear energy.
Explanation
- Gamma rays are the most penetrating; alpha the least.
- Nuclear reactors use controlled fission.
- The Sun’s energy comes from fusion.
- Semiconductors and lasers rest on quantum physics.
- Radioisotopes are used in medicine and dating.
Step by step
- Identify the concept, law or fact asked.
- Recall the definition, formula, example or year.
- Apply it and eliminate mismatched options.
Examples
- After two half-lives, one-quarter of a radioactive sample remains.
- Light ejecting electrons from a metal → photoelectric effect.
- Most penetrating radiation → gamma rays.
Traps and advanced points
- Trap: Giving Einstein’s Nobel to relativity.
- Trap: Mixing alpha and beta.
- Trap: Thinking fusion powers reactors today.
Quick revision
- Photoelectric effect (1921 Nobel).
- α, β, γ; half-life.
- E = mc²; fission vs fusion.
Practice MCQs
Einstein received the Nobel Prize (1921) for explaining the —
- gravity
- theory of relativity
- photoelectric effect
- atomic bomb
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Answer: (c) photoelectric effect. Photoelectric effect.
The small dense nucleus was shown by —
- Bohr
- Rutherford
- Thomson
- Chadwick
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Answer: (b) Rutherford. Gold-foil experiment.
Fixed energy levels in atoms were proposed by —
- Bohr
- Rutherford
- Dalton
- Einstein
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Answer: (a) Bohr. Bohr model.
X-rays were discovered by —
- Röntgen
- Curie
- Faraday
- Becquerel
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Answer: (a) Röntgen. Röntgen.
Radioactivity was discovered by —
- Henri Becquerel
- Röntgen
- Rutherford
- Newton
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Answer: (a) Henri Becquerel. Becquerel.
Alpha particles are —
- neutrons
- electrons
- helium nuclei
- photons
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Answer: (c) helium nuclei. Helium nuclei.
Beta particles are —
- photons
- electrons
- helium nuclei
- protons
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Answer: (b) electrons. Electrons.
Gamma rays are —
- electrons
- electromagnetic waves
- helium nuclei
- protons
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Answer: (b) electromagnetic waves. High-energy EM waves.
Half-life is the time for —
- all nuclei to decay
- half the radioactive nuclei to decay
- one nucleus to decay
- no decay
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Answer: (b) half the radioactive nuclei to decay. Half.
A sample of 80 g with a half-life of 2 days has how much left after 6 days?
- 40 g
- 20 g
- 10 g
- 0 g
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Answer: (c) 10 g. 80 → 40 → 20 → 10.
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