SSC CGL - Detailed Guide 2025
Self-Paced Course
Modern Physics
Reference: Lucent GK, NCERT Class 6–12
1. Atomic Structure (Rutherford & Bohr Models)
Rutherford's Model (Gold Foil Experiment – 1911)
| Observation | Inference |
|---|---|
| Most alpha particles passed through | Atom is mostly empty space |
| Few deflected at angles | Nucleus is small, dense, positively charged |
| Very few rebounded | Nucleus contains most of the mass |
Drawbacks:
- Couldn't explain stability of atom (accelerating electrons should radiate energy & collapse)
Bohr's Model (1913)
- Electrons revolve in fixed orbits (energy levels/shells)
- No radiation in stable orbits
- Energy absorbed/emitted when electrons jump between levels
| Concept | Description |
|---|---|
| Shells | K, L, M, N (or n = 1, 2, 3, 4...) |
| Energy Difference | ΔE = E₂ − E₁ = hν (quantum of radiation) |
| Stability | Orbits are quantized → stable atoms |
Bohr's model explains hydrogen atom spectrum well
2. Radioactivity: Alpha, Beta, Gamma
| Radiation Type | Symbol | Charge | Mass | Penetration | Affected by Magnetic Field |
|---|---|---|---|---|---|
| Alpha (α) | ⁴₂He | +2 | 4 amu | Low | Yes |
| Beta (β) | e⁻/e⁺ | –1/ +1 | ~1/1836 amu | Medium | Yes |
| Gamma (γ) | γ | 0 | 0 (pure energy) | Very High | No |
- Radioactive decay is spontaneous and unaffected by temperature/pressure
- Measured by Geiger-Müller counter
- Alpha – least penetrating, most damaging to tissues
- Beta – medium penetration
- Gamma – high penetration, needs lead shielding
3. Nuclear Energy: Fission and Fusion
| Process | Description | Example |
|---|---|---|
| Nuclear Fission | Heavy nucleus splits into smaller nuclei, releasing energy | 235U + n → Ba + Kr + energy |
| Nuclear Fusion | Two light nuclei combine to form heavier nucleus + energy | ²H + ³H → ⁴He + n + energy |
Fission:
- Used in nuclear reactors and atomic bombs
- Controlled chain reaction in reactors
Fusion:
- Powers the sun and hydrogen bomb
- Requires extremely high temperature (thermonuclear reaction)
Fusion releases more energy than fission but is harder to achieve on Earth.
4. X-rays and Uses of Lasers
X-rays:
| Feature | Description |
|---|---|
| Discovered by | Wilhelm Roentgen (1895) |
| Nature | High energy electromagnetic wave |
| Penetration | Can pass through soft tissues, blocked by dense matter |
| Uses | Medical imaging, CT scans, airport security, cancer therapy |
Lasers (Light Amplification by Stimulated Emission of Radiation):
| Property | Description |
|---|---|
| Type of Light | Monochromatic (single wavelength), coherent |
| Direction | Highly directional (no spreading) |
| Energy Concentration | High intensity |
| Uses | Surgery (eye, skin), barcode scanning, data storage, communication |
Laser surgery is highly precise and minimally invasive
Optical fiber communication uses laser due to its coherence & directionality
Developed By Jan Mohammad
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