Modern Physics and Electronics PYQs – Physics, SSC & Railway

Answer: Option 3 — A changing electric field produces a magnetic field and vice versa, enabling self-sustaining wave propagation in a vacuum.
Maxwell showed that a changing electric field creates a magnetic field (his displacement-current term) and a changing magnetic field creates an electric field (Faraday's law).
The two fields keep regenerating each other, so the wave carries itself through empty space — no medium is needed.
Revision note — Maxwell's equations and electromagnetic waves
• James Clerk Maxwell unified electricity, magnetism and light in 1864–65; his four equations are Gauss's law for electricity, Gauss's law for magnetism, Faraday's law and the Ampère–Maxwell law.
• The equations predict waves moving at c = 1/√(μ₀ε₀) ≈ 3 × 10⁸ m/s — exactly the measured speed of light, so light is an electromagnetic wave.
• Heinrich Hertz produced and detected radio waves in 1887, confirming the prediction; Marconi later used them for wireless telegraphy.
• EM waves are transverse: the electric and magnetic fields oscillate at right angles to each other and to the direction of travel; they can be polarised, which proves the transverse nature.
• The electromagnetic spectrum in increasing frequency: radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays.
• All of them travel at c in vacuum and carry energy in photons, E = hν.
• Sunlight reaches the Earth by radiation across the vacuum of space — the everyday proof that EM waves need no medium.
Do not mix these up
• Electromagnetic waves vs sound waves — sound is a mechanical, longitudinal wave that needs a medium and moves at about 340 m/s in air; EM waves are transverse and cross a vacuum at 3 × 10⁸ m/s.
• Maxwell's equations vs Maxwell's corkscrew rule — the rule is only a memory aid for the direction of the magnetic field round a current-carrying wire.
Also asked from this topic: how the Sun's heat crosses empty space (radiation); what proves light is transverse (a Polaroid); the other name of the right-hand thumb rule (Maxwell's corkscrew rule).

Answer: Option 3 — It states that the more precisely we know a particle's position, the less precisely we can know its momentum, affecting subatomic behavior.
The principle says position and momentum cannot both be known exactly at the same time: Δx × Δp ≥ h/4π.
The limit is built into nature, not into the instrument, and it matters only at the subatomic scale where h is not negligible.
Revision note — Heisenberg's Uncertainty Principle
• Stated by Werner Heisenberg (Germany) in 1927; he had already founded matrix mechanics in 1925 and received the Nobel Prize in Physics for 1932.
• Form: Δx · Δp ≥ h/4π (that is, ≥ ħ/2), where h = 6.626 × 10⁻³⁴ J s is Planck's constant; a similar relation links energy and time, ΔE · Δt ≥ h/4π.
• Consequence: an electron in an atom has no definite path or orbit; the Bohr orbit gives way to the orbital, a region of probability described by Schrödinger's wave equation (1926).
• It also explains why a particle confined in a small space has a large momentum spread, which gives the zero-point energy and lets alpha particles tunnel out of the nucleus.
• For everyday objects the uncertainties are so tiny that classical mechanics works perfectly — a satellite's position and velocity are both known to high accuracy.
• Other founders of quantum theory: Max Planck (quantum of energy, 1900), Einstein (photoelectric effect, 1905), de Broglie (matter waves, 1924), Schrödinger (wave equation, 1926).
Do not mix these up
• Uncertainty principle vs Pauli exclusion principle — Heisenberg limits the joint precision of position and momentum; Pauli says no two electrons in an atom share all four quantum numbers.
• Uncertainty vs observer effect — the uncertainty is a property of the particle itself, not merely the disturbance caused by looking at it.
Also asked from this topic: the scientist associated with the theory of quantum mechanics published in 1925 (Werner Heisenberg); the form of energy carried by a photon (electromagnetic energy, E = hν).

Answer: Option 3 — Light energy is quantized and delivered in discrete packets, confirming the particle nature of light.
Electrons are emitted only above a threshold frequency, and their maximum kinetic energy rises with frequency, not intensity.
A wave picture cannot explain either result; photons of energy E = hν can.
Revision note — Photoelectric effect
• Observed by Heinrich Hertz in 1887 and explained by Albert Einstein in 1905, who won the Nobel Prize in Physics in 1921 for this work.
• Einstein's photoelectric equation: hν = φ + K.max, so K.max = h(ν − ν₀), where φ is the work function and ν₀ the threshold frequency.
• Intensity decides only how many electrons come out — the photocurrent; frequency decides how fast they come out.
• Below the threshold frequency no electron is emitted, however bright the light, and emission is instantaneous (about 10⁻⁹ s).
• Planck's constant h = 6.626 × 10⁻³⁴ J s; a photon has zero rest mass, energy E = hν and momentum p = h/λ.
• R. A. Millikan verified the equation experimentally in 1916; the effect is used in photocells, solar cells, burglar alarms and light meters.
Do not mix these up
• Intensity vs frequency — raising the intensity raises the number of photoelectrons; raising the frequency raises their kinetic energy.
• Photoelectric effect vs thermionic emission — photoelectrons are freed by light, thermionic electrons by heating the metal. Neither creates electrons; both only release ones already present.
Also asked from this topic: what the photoelectric effect proves (the particle nature of light); whether the kinetic energy depends on intensity (no — on frequency); the rest mass of a photon (0 kg).

Answer: Option 3 — The speed of light is constant in all inertial frames, leading to time dilation and length contraction
Special relativity begins from c being the same for every inertial observer.
From that one postulate follow time dilation — a moving clock runs slow — and length contraction, a shrinking of moving lengths along the direction of motion.
Revision note — Special Theory of Relativity
• Published by Albert Einstein in 1905, his Annus Mirabilis, in the paper On the Electrodynamics of Moving Bodies.
• Postulate 1: the laws of physics are identical in all inertial frames.
• Postulate 2: the speed of light in vacuum, c = 3 × 10⁸ m/s, is the same for every observer, whatever the motion of the source or the observer.
• Consequences: time dilation, length contraction, relativity of simultaneity, relativistic mass increase, and mass–energy equivalence, E = mc².
• Evidence the boards like: muons formed high in the atmosphere reach the ground only because of time dilation, and GPS satellite clocks must be corrected for relativistic effects.
• Nothing carries a signal faster than c. Gravitational waves travel at exactly the speed of light, as confirmed by LIGO, whose detection won the 2017 Nobel Prize in Physics.
Do not mix these up
• Special vs General relativity — Special (1905) deals with inertial frames at constant velocity; General (1915) adds acceleration and gravity, treating gravity as the curvature of spacetime.
• What is absolute — relativity makes space and time relative to the observer and only the speed of light absolute. It is Newtonian physics that held space and time to be absolute.
• Mass and energy — they are not separate conserved quantities. E = mc² makes them two forms of one thing.
Also asked from this topic: who developed the General Theory of Relativity (Albert Einstein); the year of the General Theory (1915).

Answer: Option 1 — A remains unchanged, Z decreases by 1
In beta-plus (positron) decay a proton turns into a neutron, throwing out a positron and a neutrino.
The number of nucleons is the same as before, but the nucleus has one positive charge less — so A is unchanged and Z falls by 1.
Revision note — Radioactive decay
• Alpha decay: the nucleus emits a helium nucleus (2 protons + 2 neutrons), so A decreases by 4 and Z decreases by 2.
• Beta-minus decay: a neutron becomes a proton, emitting an electron and an antineutrino — A unchanged, Z increases by 1.
• Beta-plus decay: a proton becomes a neutron, emitting a positron and a neutrino — A unchanged, Z decreases by 1.
• Gamma decay: only energy leaves the nucleus, so both A and Z stay unchanged.
• Gamma rays have the greatest penetrating power and alpha particles the least; alpha has the greatest ionising power.
• Radioactivity was discovered by Henri Becquerel in 1896. The SI unit is the becquerel (Bq), and 1 curie = 3.7 × 10¹⁰ Bq.
• Half-life is the time in which half of a sample decays; after two half-lives only 25 per cent of the original sample is left.
• Wolfgang Pauli predicted the neutrino in 1930–31 to account for the energy that seemed to go missing in beta decay.
Do not mix these up
• Beta-minus vs beta-plus — beta-minus raises Z by 1 and emits an electron; beta-plus lowers Z by 1 and emits a positron. The mass number changes in neither.
• Beta-plus vs electron capture — both lower Z by 1, but in electron capture the nucleus absorbs an inner-shell electron instead of emitting a positron.
Also asked from this topic: what beta-plus decay does (Z falls by 1, A unchanged, SSC CGL 2025); the decay that raises Z by one (beta-minus decay, SSC CGL 2025); the decay that alters Z but not A (beta decay, SSC CGL 2025); who predicted the neutrino (Wolfgang Pauli, RRB NTPC 2025).

Correct Answer:
Correct Answer: Option 3 – They are absorbed by upper atmosphere
Explanation:
Gamma rays (and X-rays, most UV) from the Sun areabsorbed by the upper layers of the atmosphere(ozone layer and ionosphere), so they do not reach the Earth's surface in significant amounts. This absorption protects life on Earth from their harmful, high-energy ionizing radiation.
Important Facts:
• Gamma rays have the shortest wavelength and highest frequency/energy in the EM spectrum (not long wavelength).
• The ozone layer mainly absorbs harmful UV radiation; the upper atmosphere absorbs gamma/X-rays.
• Radio waves and visible light pass through the atmosphere with little absorption.

Answer: Option 1 — Ultraviolet part of the spectrum
Concept: UV radiationdamages the DNA of bacteria and microbes, killing them. UV lamps are widely used to disinfect and purify water → Option 1.
(Gamma rays are used for sterilising medical equipment/food, but common water purifiers use UV.)

Topic: Nature of Light Energy
Photonis a quantum (packet) of electromagnetic radiation.
Light exhibits wave-particle duality and travels as electromagnetic waves.
Electromagnetic spectrum includes: radio waves, microwaves, infrared, visible light, UV, X-rays, gamma rays.
Photon energy:E = hν(h = Planck's constant, ν = frequency)
Answer: Option 1 → Electromagnetic Energy

Correct Answer: Hydrogen and its isotopes — Option (c)
Nuclear fusion reactions commonly use Hydrogen and its isotopes — specifically Deuterium (²H or D) and Tritium (³H or T). In fusion, light nuclei combine to form heavier nuclei, releasing enormous energy (E = mc²). The most common fusion reaction is: Deuterium + Tritium → Helium-4 + Neutron + Energy (17.6 MeV). The Sun and other stars produce energy through the proton-proton chain (fusion of hydrogen isotopes). Hydrogen bombs (thermonuclear weapons) use deuterium and tritium. ITER (International Thermonuclear Experimental Reactor) in France uses D-T fusion. Hydrogen isotopes are used because they have the lowest Coulomb barrier for fusion among all elements.
Option (a) Uranium and Thorium — Incorrect: Uranium and Thorium are heavy elements used in nuclear FISSION reactions (where heavy nuclei split into lighter ones), not fusion. U-235 is used in nuclear reactors.
Option (b) Carbon and Nitrogen — Incorrect: While carbon and nitrogen participate in the CNO cycle (Carbon-Nitrogen-Oxygen cycle) in stars as catalysts for hydrogen fusion, they themselves are not the primary fuel for fusion.
Option (d) Oxygen and Helium — Incorrect: These are products or byproducts of various nuclear reactions; the primary fuel for nuclear fusion reactions is Hydrogen and its isotopes (Deuterium and Tritium).

0 kg is correct. According to the Standard Model of particle physics and Einstein's theory of special relativity, photons are massless particles — they have zero rest mass. This is why photons can travel at the speed of light (c = 3×10⁸ m/s); only massless particles can achieve this speed. Although photons carry energy (E = hf) and momentum (p = h/λ), their rest mass is exactly zero.
• 1 kg: Photons have no measurable mass; 1 kg is an absurdly large macroscopic mass with no relevance to subatomic particles.
• 9.1×10⁻³¹ kg: This is the rest mass of an electron, not a photon.
• 1.6×10⁻¹⁹ kg: This value (approximately) is sometimes confused with the elementary charge (1.6×10⁻¹⁹ coulombs); it is not a mass value associated with photons.

Correct Answer: Option 2 – All objects emit infrared radiation proportional to their temperature
Every object with a temperature above absolute zero emits infrared (IR) radiation. The amount and wavelength of IR radiation emitted is directly proportional to the object's temperature (as described by Planck's Law and Wien's Displacement Law). This means:
• Warm objects (humans, vehicles, engines) emit more IR than their cooler surroundings.
• IR cameras detect this temperature difference, making them excellent for surveillance (night vision), search-and-rescue, medical diagnostics, and military thermal imaging.
• Unlike visible light imaging, IR imaging does not need external light sources — it works in complete darkness.
Why other options are wrong:
Option 1 (IR waves have higher energy): Infrared waves actually have LOWER energy than visible light (longer wavelength, lower frequency).
Option 3 (Not absorbed by atmospheric gases): Infrared is partially absorbed by atmospheric gases (especially CO2 and water vapour ). It is not ideal for interstellar communication.
Option 4 (IR can ionize atoms): Infrared radiation does NOT have enough energy to ionize atoms. Ionizing radiation includes X-rays, gamma rays, and UV (high energy).

Correct Answer: Option 2 – Fusion releases more energy and produces fewer long-lived radioactive wastes
Nuclear Fusion combines light nuclei (e.g., deuterium and tritium — hydrogen isotopes) to form a heavier nucleus, releasing enormous amounts of energy — far more per unit mass than fission. Fusion also produces significantly fewer long-lived radioactive by-products compared to fission, making it a cleaner energy source in theory.
Nuclear Fission splits heavy nuclei (like uranium-235 or plutonium-239) and produces considerable long-lived radioactive waste.
Why other options are wrong:
Option 1 (Fusion releases less energy): Incorrect. Fusion releases MORE energy per unit mass than fission.
Option 3 (Fission uses hydrogen, fusion uses uranium): This is completely reversed. Fusion uses hydrogen isotopes (deuterium/tritium); fission uses heavy atoms like uranium.
Option 4 (Fission requires plasma temperatures): This is reversed. Fusion requires extremely high plasma temperatures (millions of degrees). Fission does not require plasma temperatures.

Correct Answer: 1 only
Statement 1: CORRECT. de Broglie wavelength λ = h/p (where h = Planck's constant, p = momentum). As momentum increases, wavelength decreases (inverse relationship). This is a fundamental principle.
Statement 2: INCORRECT. The de Broglie wavelength is negligibly small for macroscopic objects (because their mass is large, making λ = h/mv extremely tiny). It is significant only for subatomic particles like electrons, protons, etc.
Statement 3: INCORRECT. Wave-particle duality applies to ALL matter and radiation — photons, electrons, protons, and even macroscopic particles (though negligibly small). It is not limited to photons.
Therefore, only Statement 1 is correct.

Beta-minus (β−) decay: a neutron is converted to a proton, electron and antineutrino.
Proton number (Z) increases by 1, mass number (A) remains unchanged.
Alpha decay reduces both A and Z; Beta-plus decreases Z by 1; Gamma decay changes neither.

In beta decay, a neutron is converted to a proton (β−) or vice-versa (β+).
The atomic number changes by ±1 but the mass number remains unchanged.
Alpha decay changes both A and Z; gamma decay changes neither.

In beta-plus (β+) decay, a proton is converted into a neutron with the emission of a positron and a neutrino.
Hence the atomic (proton) number decreases by 1, while the mass number remains unchanged.

In fission a small amount of mass is converted into energy (mass defect).
By E = mc², this lost mass appears as released energy.
Hence the products have slightly less mass than the reactants.

• Statement 1: INCORRECT — Kinetic energy of photoelectrons depends on the frequency of light, NOT intensity. Greater intensity means more electrons, not more energetic ones.
• Statement 2: CORRECT — The photoelectric effect proved that light behaves as particles (photons), as demonstrated by Einstein (Nobel Prize 1921).
• Statement 3: CORRECT — Below threshold frequency (v₀), no photoelectric effect occurs regardless of how intense the light is.
• Answer: Statements 2 and 3 are correct.

Correct Answer: Option 3 – 20 days
Half-life (t½) = 10 days. If 75% decays, 25% remains.
After 1 half-life (10 days): 50% remains (50% decayed).
After 2 half-lives (20 days): 25% remains (75% decayed).
So, for 75% of the sample to decay, 25% must remain, which takes exactly 2 half-lives = 2 × 10 = 20 days.
Verification: N/N₀ = (1/2)^n where n = number of half-lives. 0.25 = (0.5)^n → n = 2. Time = 2 × 10 = 20 days. ✓
Option 1 (5 days): After 5 days (half a half-life), approximately 29% has decayed — far less than 75%.
Option 2 (10 days): After 1 half-life, 50% decays — not 75%.
Option 4 (30 days): After 3 half-lives, 87.5% decays — more than 75%.

Correct Answer: Option 4 — Alpha particles
An alpha (α) particle is a doubly-charged helium ion, He²⁺ — a helium atom that has lost both of its electrons. It therefore consists of 2 protons and 2 neutrons, carries a charge of +2 and has a mass of about 4 u, making it identical to the nucleus of a helium-4 atom.
Alpha particles are given out in radioactive alpha decay, in which the mass number falls by 4 and the atomic number by 2. Because they are heavy and doubly charged, they are the least penetrating of the three nuclear radiations — a sheet of paper or a few centimetres of air stops them — but they are the most ionising.
These are exactly the particles Rutherford fired at a thin gold foil in his famous scattering experiment that revealed the nucleus.
❌ Why the other options are incorrect
✗ Gamma particles — gamma (γ) rays are high-energy electromagnetic waves, not particles of matter; they have no mass and no charge.
✗ X-rays — also electromagnetic radiation, discovered by W. C. Roentgen; they are produced outside the nucleus and carry no charge.
✗ Beta particles — beta (β) particles are fast-moving electrons with a single negative charge (−1), released when a neutron changes into a proton inside the nucleus.
📌 SSC & Railway Exam Facts
• α = He²⁺ (helium nucleus), β = electron, γ = electromagnetic wave.
• Penetrating power: γ > β > α; ionising power: α > β > γ.
• α is stopped by paper, β by an aluminium sheet, γ needs thick lead or concrete.
• Radioactivity was discovered by Henri Becquerel; the α, β and γ rays were named by Rutherford (and γ by Villard).
• In α-decay: mass number −4, atomic number −2; in β-decay: mass number unchanged, atomic number +1.