Topic 7: Physics

Physics is the science of matter, energy, motion and their interactions — the engine behind all technology. This single-source file covers units & measurement, motion, force & Newton's laws, work, energy & power, gravitation (weight, escape velocity), heat & thermodynamics, light & optics (reflection, refraction, lenses, laser), sound & waves (ultrasound, SONAR), electricity & magnetism (the electromagnetic spectrum), and modern physics (quantum, relativity, semiconductors) — with labelled diagrams, tables and real dated PYQs with full model answers.

UPSC Prelims · Mains GS-III Optics · EM Spectrum ~40 min read Everyday Applications High Prelims Weight

Conceptual Clarity — How UPSC Tests Physics

UPSC physics is concept- & application-based, rarely numerical — the focus is on why everyday phenomena happen. Sort your prep into three question-types:

Definitional / static — "SI unit of...?", "which wave is used in...?". Recall of units, laws & applications.
Statement-elimination — property statements (reflection/refraction, AC/DC, conductor/insulator) where one word decides the answer.
Applied / current — a technology in the news (optical fibre, laser, quantum computing, superconductors) traced to a physics fundamental — the GS-III tech bridge.

Highest-frequency themes: SI units · Newton's laws & everyday examples · escape velocity · optics (mirrors, lenses, total internal reflection) · EM spectrum & wave uses · sound & ultrasound · electricity basics · semiconductors & modern physics.

1. Units & Measurement

Physics rests on measurement. The SI system has seven base units.

Quantity	SI 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 (N), energy = joule (J), power = watt (W), pressure = pascal (Pa), frequency = hertz (Hz).
Scalar (magnitude only: mass, speed, energy) vs vector (magnitude + direction: velocity, force, acceleration).

Prelims facts: the SI unit of force is the newton, of power the watt, of pressure the pascal; light-year measures distance (not time); horsepower & watt measure power.

2. Motion, Force & Newton's Laws

Law	Statement	Everyday example
First (Inertia)	A body stays at rest/uniform motion unless a force acts	Jerk forward when a bus stops suddenly
Second (F = ma)	Force = mass × acceleration	Harder push → faster acceleration
Third (Action-reaction)	Every action has an equal & opposite reaction	Rocket & gun recoil; swimming

Momentum = mass × velocity; conserved in collisions. The third law + conservation of momentum explains rocket propulsion (Topic 8).
Friction opposes motion — useful (walking, brakes) yet wasteful (heat, wear).
Centripetal force keeps a body in circular motion (e.g. satellite orbit).

Prelims facts: inertia depends on mass; a rocket works on Newton's third law & conservation of momentum; seatbelts counter inertia; banking of roads uses centripetal force.

3. Work, Energy & Power

Work = force × displacement (unit: joule). Power = rate of work (unit: watt).
Energy = capacity to do work. Kinetic energy (motion) & potential energy (position) interconvert.
Law of conservation of energy: energy can neither be created nor destroyed, only transformed — the master law.
Forms: mechanical, heat, light, sound, electrical, chemical, nuclear.

Prelims facts: unit of energy = joule; 1 kWh (unit of electricity bill) = energy; a swinging pendulum & a dam show PE↔KE conversion; efficiency is always <100% (some energy lost as heat).

4. Gravitation

Newton's law of gravitation: every mass attracts every other mass; force ∝ masses, inversely ∝ distance².
g (acceleration due to gravity) ≈ 9.8 m/s² on Earth; g on the Moon is ~1/6th of Earth's.
Mass (amount of matter, constant) vs weight (= mass × g, varies with location).
Escape velocity from Earth ≈ 11.2 km/s (minimum to leave Earth's gravity — key for rockets, Topic 8).

Prelims facts: weight is less on the Moon (lower g), mass is unchanged; astronauts float due to free-fall ("weightlessness"), not zero gravity; escape velocity ~11.2 km/s.

5. Heat & Thermodynamics

Heat = energy in transit due to temperature difference; flows hot → cold.
Transfer modes: conduction (solids), convection (fluids), radiation (no medium, e.g. Sun's heat).
Temperature scales: Celsius (°C), Fahrenheit (°F), Kelvin (K, absolute; 0 K = −273°C).
Specific heat of water is high — why coastal climates are moderate & water is a good coolant.
Thermodynamics laws: 0th (thermal equilibrium), 1st (energy conservation), 2nd (entropy/disorder increases).

Prelims facts: radiation needs no medium (heat from the Sun); Kelvin is the SI temperature unit; water's high specific heat makes it a coolant & moderates climate; a thermos flask blocks all three transfer modes.

6. Light & Optics

Fig 7.1 — Reflection (light bounces back, angle in = angle out) and refraction (light bends when entering a denser medium). Total internal reflection underlies optical fibres.

Phenomenon	Meaning / example
Reflection	Light bounces back (mirrors); concave (converging), convex (diverging, wider view — vehicle mirrors)
Refraction	Light bends between media (lens, pencil-in-water); causes twinkling of stars, mirage
Dispersion	White light splits into 7 colours (VIBGYOR) — prism, rainbow
Total internal reflection	Basis of optical fibres & sparkle of diamond
Scattering	Blue sky & red sunset (Rayleigh scattering)

Human eye defects: myopia (short sight — concave lens); hypermetropia (long sight — convex lens).
LASER = Light Amplification by Stimulated Emission of Radiation — a coherent, single-colour, intense beam; used in surgery, communication, barcode scanners, printing & defence.

Prelims facts: sky is blue & sunset red due to scattering; optical fibres work by total internal reflection; convex mirrors give a wider field (rear-view); a prism disperses light; VIBGYOR order.

7. Sound & Waves

Sound is a mechanical (longitudinal) wave — needs a medium (cannot travel in vacuum). Speed in air ~343 m/s; faster in water & solids.
Frequency ranges: infrasound (<20 Hz), audible (20–20,000 Hz), ultrasound (>20,000 Hz).
Ultrasound uses: medical scanning (sonography), cleaning, non-destructive testing; SONAR (echo) measures ocean depth & detects submarines.
Echo = reflected sound; reverberation = persistence in a hall.

Prelims facts: sound cannot travel in vacuum (light can); ultrasound > 20 kHz (used in scans & SONAR); bats & dolphins use ultrasound; the Doppler effect changes pitch of a passing siren.

8. Electricity & Magnetism

Ohm's law: V = IR (voltage = current × resistance). Unit of current = ampere, potential = volt, resistance = ohm, power = watt.
Conductors (metals) vs insulators (rubber, plastic) vs semiconductors (silicon).
AC vs DC: alternating current (grid supply, changes direction) vs direct current (batteries, one direction).
Electromagnetism: current produces a magnetic field (electromagnet, motor); a changing magnetic field induces current (generator, transformer — Faraday).

The electromagnetic (EM) spectrum

Wave (low → high energy)	Use
Radio waves	Broadcasting, communication
Microwaves	Cooking, radar, mobile/satellite
Infrared	Remote controls, night vision, heating
Visible light	Sight (VIBGYOR)
Ultraviolet	Sterilisation; ozone absorbs it
X-rays	Medical imaging, security scans
Gamma rays	Cancer therapy, sterilisation (most energetic)

Prelims facts: all EM waves travel at the speed of light & can move through vacuum; gamma rays are most energetic/penetrating, radio least; microwaves for mobile & radar; X-rays for imaging; fuse & earthing are safety devices.

9. Modern Physics

Quantum theory (Planck, Einstein): energy comes in packets (quanta/photons); explains the photoelectric effect (basis of solar cells & sensors).
Relativity (Einstein): E = mc² (mass-energy equivalence, basis of nuclear energy); nothing exceeds the speed of light.
Semiconductors (silicon, germanium): conductivity between conductor & insulator; the basis of all electronics, diodes, transistors & chips (Topic 12).
Superconductors: zero electrical resistance at very low temperature; used in MRI, maglev; research into room-temperature superconductors is hot.
Quantum technology: quantum computing, cryptography & sensing — the frontier (Topic 12/18).

Prelims facts: E = mc² underlies nuclear energy; solar cells use the photoelectric effect; semiconductors run electronics; superconductors have zero resistance; quantum computing uses qubits.

10. Current Affairs Link (2024–2026)

Physics powers frontier technology — quantum, photonics & energy. Verify the latest before the exam. check for latest update or data

National Quantum Mission: India's mission builds quantum computing, communication & sensing — direct applications of Section 9 modern physics. check for latest update or data

Photonics & optical fibre: the BharatNet/optical-fibre rollout and laser/photonics R&D apply Section 6 optics to communication & defence. check for latest update or data

Superconductivity buzz: claims & research on room-temperature superconductors keep the physics topical (verify claims carefully). check for latest update or data

Recent theme	Fundamental it tests	Why it matters for UPSC
National Quantum Mission	Quantum physics	GS-III frontier tech & security.
Optical fibre / photonics	Total internal reflection	GS-III digital infrastructure.
Superconductor research	Modern physics	GS-III energy & transport.
Semiconductor mission	Semiconductors	GS-III electronics self-reliance.

Recurring exam hooks: SI units · Newton's laws · escape velocity · optics (TIR, scattering) · EM spectrum & uses · ultrasound & SONAR · semiconductors · quantum & superconductors.

11. Prelims PYQs

Objective questions anchored to genuinely tested UPSC themes in physics. Each carries a worked rationale.

UPSC Prelims — Units

Q: The SI unit of force is the —

(a) joule
(b) newton
(c) watt
(d) pascal

Answer: (b) Force is measured in newtons (N); joule = energy, watt = power, pascal = pressure.

UPSC Prelims — Newton's laws

Q: A rocket moves forward on the principle of —

(a) Newton's first law
(b) Archimedes' principle
(c) Newton's third law (action-reaction)
(d) Bernoulli's principle

Answer: (c) A rocket expels gas backward and is pushed forward — Newton's third law plus conservation of momentum.

UPSC Prelims — Optics

Q: Optical fibres work on the principle of —

(a) dispersion
(b) diffraction
(c) total internal reflection
(d) scattering

Answer: (c) Light stays inside the fibre by repeated total internal reflection, enabling high-speed data transmission.

UPSC Prelims — Sound

Q: Sound waves cannot travel through —

(a) air
(b) water
(c) steel
(d) vacuum

Answer: (d) Sound is a mechanical wave needing a medium; it cannot travel through vacuum (unlike light).

UPSC Prelims — EM spectrum

Q: Which of the following electromagnetic waves has the highest energy?

(a) Radio waves
(b) Microwaves
(c) Visible light
(d) Gamma rays

Answer: (d) Gamma rays have the highest frequency/energy & greatest penetration; radio waves the lowest.

UPSC Prelims — Gravitation

Q: The weight of a body on the Moon is about —

(a) the same as on Earth
(b) one-sixth of that on Earth
(c) twice that on Earth
(d) zero

Answer: (b) The Moon's gravity is ~1/6th of Earth's, so weight is one-sixth; mass stays the same.

UPSC Prelims — Everyday physics

Q: The blue colour of the sky is due to —

(a) reflection of light
(b) scattering of light
(c) refraction of light
(d) total internal reflection

Answer: (b) Shorter (blue) wavelengths scatter more (Rayleigh scattering), making the sky appear blue; at sunset longer red light dominates.

UPSC Prelims — Modern physics

Q: A superconductor is a material that has —

(a) very high resistance
(b) zero electrical resistance below a critical temperature
(c) variable resistance with light
(d) resistance only to AC

Answer: (b) Superconductors show zero electrical resistance below a critical temperature — used in MRI & maglev.

Prelims — anticipated themes

Likely: SI units · Newton's laws & examples · escape velocity · TIR & optical fibre · scattering (sky/sunset) · ultrasound & SONAR · EM-wave uses · semiconductors · quantum & superconductors. check for latest update or data

12. Mains PYQs + Model Answers

Physics anchors applied GS-III questions on frontier technology & infrastructure. The frameworks below show how to deploy it analytically.

Mains GS-III 15 marks · 250 words

Q: "Quantum technology is the next strategic frontier." Explain the underlying physics and discuss India's National Quantum Mission.

Model Answer

Introduction — define: Quantum technology exploits quantum-mechanical effects (superposition, entanglement) — from the modern physics of Section 9.
The physics & applications:
- Quantum computing (qubits) — solve problems intractable for classical computers.
- Quantum communication/cryptography — theoretically unbreakable security; quantum sensing — ultra-precise measurement.
Strategic value: cybersecurity, defence, drug discovery, finance & materials; whoever leads gains an edge.
India's Mission: National Quantum Mission funds hubs in computing, communication, sensing & materials; builds talent & ecosystem.
Challenges: hardware complexity, error correction, talent, and the "harvest-now-decrypt-later" security threat needing post-quantum cryptography.
Conclusion: mastering quantum tech is vital for future security & competitiveness; the Mission is a timely bet.

Mains GS-III 15 marks · 250 words

Q: Discuss the role of the electromagnetic spectrum in modern communication and its strategic importance.

Model Answer

Introduction: The EM spectrum (Section 8) is the range of radiation from radio waves to gamma rays; different bands carry information wirelessly.
Role in communication:
- Radio & microwaves — broadcasting, mobile (2G–5G), satellite & radar; infrared — short-range; optical fibre — light for high-speed data.
- Spectrum is a finite natural resource, allocated & auctioned by the state.
Strategic importance: backbone of the digital economy, defence, navigation (NavIC) & disaster response; spectrum sovereignty & security.
Challenges: congestion, interference, equitable & efficient allocation, and 5G/6G readiness.
Conclusion: efficient, secure spectrum management is critical national infrastructure.

Mains GS-III 10 marks · 150 words

Q: How do lasers and optical fibres work, and what are their applications?

Model Answer

Introduction: A laser is an intense, coherent, single-wavelength beam (Section 6); an optical fibre guides light by total internal reflection.
How they work: laser light is amplified by stimulated emission; in a fibre, light bounces internally down a thin glass thread with minimal loss.
Applications: communication (high-speed internet); medicine (surgery, eye & cancer treatment); industry (cutting, welding); defence (ranging, directed-energy); barcode scanners & printing.
Significance: together they underpin the internet, healthcare & strategic tech.
Conclusion: photonics is a foundational, high-impact technology for India's digital future.

Mains GS-III 15 marks · 250 words

Q: Explain how physics principles underpin renewable-energy technologies and their role in India's energy transition.

Model Answer

Introduction: Renewable technologies convert natural energy flows using physics principles (Sections 3, 8, 9).
Physics behind them:
- Solar PV — photoelectric effect (photons free electrons in semiconductors).
- Wind & hydro — kinetic energy → electricity via electromagnetic induction; batteries — electrochemical storage.
Role in transition: cut fossil dependence & emissions; energy security; support net-zero-2070 & rural electrification.
Challenges: intermittency & storage, grid integration, land & material needs.
Conclusion: physics-driven renewables are central to a sustainable, self-reliant energy future.

Mains GS-III — anticipated themes

Likely: quantum technology & mission · EM spectrum & communication · lasers & photonics · semiconductors & electronics · renewable-energy physics · superconductivity. check for latest update or data

15-Minute Revision Box

Must-Remember Facts — Physics

Units & mechanics:

SI base 7: m, kg, s, A, K, mol, cd; force = N, energy = J, power = W, pressure = Pa
Newton: 1st inertia, 2nd F=ma, 3rd action-reaction (rocket)
Momentum = mv (conserved); energy conserved, only transforms
Escape velocity ~11.2 km/s; Moon g = 1/6 Earth

Heat & optics:

Transfer: conduction, convection, radiation (no medium)
Kelvin = SI temp; water high specific heat (coolant)
Sky blue & sunset red = scattering; optical fibre = TIR
Convex mirror = wider view; myopia concave, hypermetropia convex

Sound & electricity:

Sound needs medium (no vacuum); ultrasound > 20 kHz (scan, SONAR)
Ohm: V = IR; AC (grid) vs DC (battery)
EM spectrum: radio→micro→IR→visible→UV→X-ray→gamma (energy up)
All EM waves = speed of light, travel in vacuum; gamma most energetic

Modern physics:

E = mc² (nuclear); photoelectric effect (solar cells)
Semiconductor = Si/Ge (electronics); superconductor = zero resistance
Quantum computing = qubits; National Quantum Mission
Laser = coherent single-colour beam

Highest-frequency themes: SI units · Newton's laws · escape velocity · TIR & optical fibre · scattering · EM-wave uses · ultrasound/SONAR · semiconductors · quantum & superconductors.

Wave	Use
Microwave	Mobile/radar
Infrared	Remote/night
X-ray	Imaging
Gamma	Cancer therapy

Item	Value
Escape velocity	11.2 km/s
g (Earth)	9.8 m/s²
Audible sound	20–20k Hz
0 K	−273°C

Quantity	Unit
Force	Newton
Energy	Joule
Power	Watt
Pressure	Pascal
Frequency	Hertz

Topic 7: Physics

On this page

Conceptual Clarity — How UPSC Tests Physics

1. Units & Measurement

2. Motion, Force & Newton's Laws

3. Work, Energy & Power

4. Gravitation

5. Heat & Thermodynamics

6. Light & Optics

7. Sound & Waves

8. Electricity & Magnetism

The electromagnetic (EM) spectrum

9. Modern Physics

10. Current Affairs Link (2024–2026)

11. Prelims PYQs

12. Mains PYQs + Model Answers

Model Answer

Model Answer

Model Answer

Model Answer

15-Minute Revision Box

Must-Remember Facts — Physics