- Physics in SSC and RRB papers is mostly applied: "why does a pencil look bent in water?" rather than derivations.
- Most questions come from five areas: units, motion, light, sound and electricity.
- Each concept below is paired with the everyday example that examiners use.
The Seven SI Base Units
| Quantity | Unit | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Electric current | ampere | A |
| Temperature | kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
Derived units: force → newton; work and energy → joule; power → watt; pressure → pascal; frequency → hertz; charge → coulomb; potential difference → volt; resistance → ohm. A light-year is a unit of distance, not time.
Newton's Laws With Examples
- First law (inertia): passengers jerk forward when a bus brakes suddenly; dust leaves a carpet when it is beaten.
- Second law (F = ma): a cricketer pulls the hands back while catching, increasing the time and reducing the force.
- Third law (action–reaction): a gun recoils; a rocket moves up as gases rush down; we walk by pushing the ground backwards.
Speed of light3 × 10⁸ m/s
Sound in air~343 m/s at 20°C
Audible range20 Hz – 20 kHz
Indian mains230 V, 50 Hz
Same in °C and °F−40
Water densest at4°C
Light Phenomena
| Phenomenon | Everyday example |
|---|---|
| Refraction | A pencil looks bent in water; a pool looks shallower than it is; stars twinkle |
| Total internal reflection | Optical fibres, the sparkle of diamonds, mirages |
| Dispersion | A prism splits white light; a rainbow |
| Scattering | The sky is blue; sunrise and sunset look red; danger signals are red because red scatters least |
| Interference | Colours on soap bubbles and oil films |
C.V. Raman discovered the Raman effect (scattering of light by molecules) on February 28, 1928 — now National Science Day — and won the Nobel Prize in Physics in 1930.
Mirrors and Lenses
- Concave mirror: shaving and make-up mirrors, dentists' mirrors, torch and headlight reflectors, solar cookers.
- Convex mirror: rear-view mirrors in vehicles — it gives a wider field of view.
- Myopia (short-sightedness) is corrected with a concave lens; hypermetropia (long-sightedness) with a convex lens; presbyopia often needs bifocal lenses.
Sound
- Sound needs a medium and cannot travel in a vacuum. It is fastest in solids, slower in liquids and slowest in gases.
- Humans hear 20 Hz to 20,000 Hz. Above that is ultrasound (used by bats, SONAR and medical scans); below is infrasound (produced by earthquakes and used by elephants and whales).
- Loudness is measured in decibels (dB); pitch depends on frequency. An echo needs the reflecting surface to be at least about 17 m away.
Electricity and Heat
- Ohm's law: V = IR. A fuse wire has a low melting point and melts to break the circuit when the current is too high. Household supply in India is 230 V AC at 50 Hz.
- Bulb filaments are made of tungsten, which has a very high melting point. Silver is the best conductor of electricity; copper is used because it is cheaper.
- Water is densest at 4°C, so lakes freeze from the top and aquatic life survives below. −40° is the same on the Celsius and Fahrenheit scales; absolute zero is 0 K = −273.15°C.
Inventions and Discoveries
| Invention / discovery | By | Year |
|---|---|---|
| Telephone | Alexander Graham Bell | 1876 |
| Practical electric bulb | Thomas Edison | 1879 |
| X-rays | Wilhelm Röntgen | 1895 |
| Radioactivity | Henri Becquerel | 1896 |
| Electron | J.J. Thomson | 1897 |
| Neutron | James Chadwick | 1932 |
- Phenomenon ↔ example: "The sparkling of a diamond is due to…" (total internal reflection).
- Unit questions: "Which is NOT an SI base unit?" or "A light-year measures…" (distance).
- Applied optics: which mirror for rear-view, which lens for myopia.
Q1. The sparkling of a diamond is mainly due to:
✅ Answer: (b) Total internal reflection
💡 Explanation: Diamond's high refractive index causes repeated total internal reflection inside it.
Q2. Which mirror is used as a rear-view mirror in vehicles?
✅ Answer: (a) Convex
💡 Explanation: A convex mirror gives an upright, smaller image over a wider field of view.
Q3. Which of these is NOT an SI base unit?
✅ Answer: (d) Newton
💡 Explanation: Newton is a derived unit (kg·m/s²).
Q4. Myopia (short-sightedness) is corrected using a:
✅ Answer: (c) Concave lens
💡 Explanation: A concave (diverging) lens moves the image back onto the retina.
Q5. The sky appears blue because of:
✅ Answer: (d) Scattering
💡 Explanation: Shorter blue wavelengths are scattered much more by air molecules (Rayleigh scattering).
🔁 Revise in 60 Seconds
- 7 SI base units: m, kg, s, A, K, mol, cd.
- Blue sky → scattering; diamond sparkle → TIR; rainbow → dispersion; bent pencil → refraction.
- Concave lens for myopia, convex for hypermetropia; convex mirror for rear view.
- Sound fastest in solids; 20 Hz–20 kHz audible; India 230 V, 50 Hz.
📌 Prelims Traps to Avoid
- ✘ A light-year is a unit of distance.
- ✘ Sound cannot travel in a vacuum; light can.
- ✘ Stars twinkle because of refraction in the atmosphere, not scattering.
What are the seven SI base units?
Metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance) and candela (luminous intensity).
Why does the sky look blue?
Sunlight is scattered by gas molecules in the air, and shorter wavelengths like blue are scattered much more than longer ones like red. This is called Rayleigh scattering.