📑 Topics on this page (Jump to Topic):
📌 1. Electromagnetic Spectrum
All EM waves travel at 3 × 10⁸ m/s in vacuum; as wavelength falls, frequency and energy rise (E = hf). They need no medium.
| Wave (increasing energy →) | Discovery / source | Uses |
|---|---|---|
| Radio waves | Hertz (experiment) | Radio, TV, mobile communication |
| Microwaves | — | Radar, microwave oven, satellite links |
| Infrared | William Herschel | Remote control, night vision, thermal imaging; greenhouse effect |
| Visible light | — | Vision, photosynthesis |
| Ultraviolet | Ritter | Water purification, vitamin D; blocked by ozone; can cause skin cancer |
| X-rays | Röntgen (1895) | Bone fractures, baggage scanning, crystal structure |
| Gamma rays | Radioactive nuclei | Cancer treatment, sterilisation |
- Photoelectric effect: emission of electrons when light falls on a metal — explained by Einstein (Nobel 1921); basis of solar cells and photocells.
- Einstein's mass–energy relation E = mc² — the basis of nuclear energy.
📌 2. Radioactivity
- Discovered by Henri Becquerel (1896, uranium); Marie and Pierre Curie found polonium and radium. Units: becquerel (SI), curie (old).
- Half-life = time for half the nuclei to decay; unaffected by temperature or pressure. Carbon-14 half-life ≈ 5730 years — used for dating fossils/archaeological samples.
- Uses: cobalt-60 (cancer), iodine-131 (thyroid), uranium/lead method (age of rocks), Geiger–Müller counter detects radiation.
| Ray | Nature | Charge | Penetration |
|---|---|---|---|
| Alpha (α) | Helium nucleus | +2 | Least (stopped by paper) |
| Beta (β) | Electron | −1 | Medium (aluminium foil) |
| Gamma (γ) | EM wave (photon) | Zero | Highest (thick lead) |
📌 3. Nuclear Fission, Fusion & Reactors
| Fission | Fusion | |
|---|---|---|
| What happens | A heavy nucleus (U-235) splits | Light nuclei (hydrogen) join to form helium |
| Discovery | Otto Hahn & Strassmann (1938) | — |
| Example | Atom bomb, nuclear reactor | Energy of Sun and stars, hydrogen bomb |
| Condition | Chain reaction by neutrons | Extremely high temperature (~10⁷ K) |
- Reactor parts: fuel (U-235/plutonium), moderator (heavy water D₂O, graphite — slows neutrons), control rods (cadmium/boron — absorb neutrons), coolant (water, liquid sodium).
- India's first reactor Apsara (1956, Trombay); first nuclear power station Tarapur (1969). Homi J. Bhabha — father of India's nuclear programme.
📌 4. Semiconductors & Electronics
- Semiconductors: silicon, germanium; conductivity rises with temperature. Adding impurity = doping: pentavalent (phosphorus, arsenic) → n-type; trivalent (boron, gallium) → p-type.
- p-n junction diode — current in one direction (rectifier); LED — light-emitting diode; transistor (1947, Bell Labs) — amplifier and switch; an IC (chip) holds millions of transistors.
- LASER = Light Amplification by Stimulated Emission of Radiation — monochromatic, coherent, very narrow beam; eye surgery, barcodes, CD/DVD, optical-fibre communication. Theodore Maiman (1960).
- RADAR = Radio Detection And Ranging — uses radio/microwaves to find distance and direction of aircraft etc.
⚠️ Exam Traps:
- γ-rays carry no charge — they are not deflected by electric or magnetic fields.
- The Sun's energy comes from fusion, not fission.
- The moderator slows neutrons; control rods absorb them.
📌 5. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. The source of the Sun's energy is: Practice
Correct answer: (D)
Hydrogen nuclei fuse to form helium.
Hydrogen nuclei fuse to form helium.
Q2. X-rays were discovered by: Practice
Correct answer: (B)
In 1895; he won the first Nobel Prize in Physics (1901).
In 1895; he won the first Nobel Prize in Physics (1901).
Q3. Radioactivity was discovered by: Practice
Correct answer: (D)
In 1896 using uranium salts.
In 1896 using uranium salts.
Q4. Which has the highest penetrating power? Practice
Correct answer: (A)
Gamma rays need thick lead to stop them.
Gamma rays need thick lead to stop them.
Q5. In a nuclear reactor the moderator used is: Practice
Correct answer: (D)
Heavy water/graphite slow neutrons; cadmium/boron are control materials.
Heavy water/graphite slow neutrons; cadmium/boron are control materials.
Q6. Which isotope is used in carbon dating? Practice
Correct answer: (C)
C-14 has a half-life of about 5730 years.
C-14 has a half-life of about 5730 years.
Q7. A TV remote control uses: Practice
Correct answer: (A)
An IR LED sends invisible infrared pulses.
An IR LED sends invisible infrared pulses.
Q8. The ozone layer protects the Earth from: Practice
Correct answer: (C)
Ozone absorbs harmful UV-B and UV-C.
Ozone absorbs harmful UV-B and UV-C.
Q9. Who gave E = mc²? Practice
Correct answer: (D)
Mass–energy equivalence from special relativity (1905).
Mass–energy equivalence from special relativity (1905).
Q10. An alpha particle is a: Practice
Correct answer: (B)
2 protons + 2 neutrons, charge +2.
2 protons + 2 neutrons, charge +2.
Q11. The atom bomb is based on: Practice
Correct answer: (C)
Uncontrolled chain reaction of U-235/Pu-239; the hydrogen bomb uses fusion.
Uncontrolled chain reaction of U-235/Pu-239; the hydrogen bomb uses fusion.
Q12. India's first nuclear reactor was: Practice
Correct answer: (C)
Commissioned in 1956 at Trombay (BARC).
Commissioned in 1956 at Trombay (BARC).
Q13. Doping silicon with phosphorus produces a: Practice
Correct answer: (B)
A pentavalent impurity donates extra electrons.
A pentavalent impurity donates extra electrons.
Q14. In LASER, "S" stands for: Practice
Correct answer: (A)
Light Amplification by Stimulated Emission of Radiation.
Light Amplification by Stimulated Emission of Radiation.
Q15. Control rods in a nuclear reactor are made of: Practice
Correct answer: (D)
They absorb excess neutrons to control the reaction.
They absorb excess neutrons to control the reaction.
Q16. A radioactive substance has a half-life of 10 days. What fraction remains after 30 days? Practice
Correct answer: (A)
Three half-lives: (1/2)³ = 1/8.
Three half-lives: (1/2)³ = 1/8.
Q17. Who got the Nobel Prize for explaining the photoelectric effect? Practice
Correct answer: (C)
Einstein's 1921 Physics Nobel was for this.
Einstein's 1921 Physics Nobel was for this.
Q18. Which isotope is used in treating cancer? Practice
Correct answer: (A)
Gamma rays from Co-60 destroy cancer cells.
Gamma rays from Co-60 destroy cancer cells.
Q19. Which is NOT deflected by electric or magnetic fields? Practice
Correct answer: (B)
Gamma rays are uncharged EM waves.
Gamma rays are uncharged EM waves.
Q20. A microwave oven heats food because microwaves: Practice
Correct answer: (B)
Water molecules absorb microwave energy and turn it into heat.
Water molecules absorb microwave energy and turn it into heat.