Physics MCQs

Question #621

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The phenomenon of total internal reflection occurs when light travels from:

A. Denser to rarer medium at angle greater than critical angle

B. Rarer to denser medium

C. Any two media at any angle

D. Denser to rarer medium at angle less than critical angle

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Correct Answer: Option A

Explanation:
Total internal reflection (TIR) requires: (1) light travels from denser to rarer medium (n₁ > n₂), and (2) angle of incidence exceeds critical angle θ_c = sin⁻¹(n₂/n₁). Option A describes refraction toward normal; B describes partial reflection/refraction; D is incorrect. Memory aid: 'TIR: denser→rarer AND i > θ_c'. This condition-based question tests optics fundamentals, frequently examined in competitive exams. Always verify both conditions for TIR; common applications include optical fibers and prisms in binoculars.

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Question #622

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The power factor in an AC circuit is defined as:

A. Cosine of phase angle between voltage and current

B. Both (a) and (c)

C. Ratio of apparent power to true power

D. Ratio of resistance to impedance

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Correct Answer: Option B

Explanation:
Power factor = cosφ, where φ is phase angle between voltage and current. Also, power factor = R/Z (resistance/impedance) for series circuits, and = true power (W) / apparent power (VA). Thus both (a) and (c) are correct definitions. Memory tip: 'Power factor = cosφ = R/Z = P/S; ideal = 1 (purely resistive)'. This definition question tests AC circuit knowledge, crucial for competitive exams. Always link power factor to energy efficiency: low power factor means more current for same power, increasing losses.

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Question #623

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The energy of a photon is directly proportional to its:

A. Frequency

B. Speed

C. Amplitude

D. Wavelength

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Correct Answer: Option A

Explanation:
Photon energy E = hν, where h is Planck's constant, ν is frequency. Thus E ∝ ν. Since c = νλ, E = hc/λ, so E ∝ 1/λ (inversely proportional to wavelength). Amplitude relates to intensity (number of photons), not individual photon energy. Speed is constant (c) in vacuum. Memory aid: 'Higher frequency (shorter wavelength) photons have more energy (e.g., gamma > radio)'. This quantum physics concept is frequently tested in competitive exams. Always use E = hν for photon energy calculations; avoid confusing wave amplitude with photon energy.

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Question #624

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A wire of resistance R is stretched to double its length. Assuming volume remains constant, the new resistance is:

A. R

B. 4R

C. R/2

D. 2R

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Correct Answer: Option B

Explanation:
Resistance R = ρL/A. Volume V = A·L constant. If L' = 2L, then A' = V/L' = V/(2L) = A/2. Thus R' = ρ(2L)/(A/2) = 4(ρL/A) = 4R. Resistance increases by factor of 4. Memory tip: 'Stretching wire: R ∝ L² when volume constant'. This proportional reasoning problem tests resistance concepts, frequently appearing in competitive exams. Always verify volume conservation assumption; if area changed independently, result would differ. This problem assesses understanding of geometric effects on electrical properties.

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Question #625

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The principle of conservation of linear momentum is a consequence of:

A. Newton's first law

B. Newton's second law

C. Newton's third law

D. Law of conservation of energy

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Correct Answer: Option C

Explanation:
Conservation of linear momentum follows from Newton's third law (action-reaction pairs) and second law. For an isolated system, internal forces cancel in pairs (third law), so net force = 0, thus dp/dt = 0 (second law), meaning momentum constant. While derivable from second law with third law, the direct foundational link is third law. Memory aid: 'Momentum conservation ⇔ no external force ⇔ internal forces cancel (third law)'. This conceptual question tests mechanics fundamentals, crucial for competitive exams. Always distinguish conservation laws: momentum (from spatial symmetry), energy (from time symmetry).

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Question #626

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The focal length of a spherical mirror is related to its radius of curvature R by:

A. f = R

B. f = 2R

C. f = R²

D. f = R/2

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Correct Answer: Option D

Explanation:
For spherical mirrors (concave or convex), focal length f = R/2, where R is radius of curvature. This holds under paraxial approximation (small angles). Derivation uses geometry of reflection and small-angle approximations. Memory tip: 'Mirror: f = R/2; Lens: 1/f = (n-1)(1/R₁ - 1/R₂)'. This fundamental relation is frequently tested in optics sections of competitive exams. Always note sign conventions: f negative for convex mirrors, positive for concave; R follows same sign as f.

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Question #627

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The phenomenon of diffraction is most pronounced when:

A. Obstacle size is comparable to wavelength

B. Wavelength is zero

C. Obstacle size is much smaller than wavelength

D. Obstacle size is much larger than wavelength

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Correct Answer: Option A

Explanation:
Diffraction (bending of waves around obstacles) is significant when obstacle or aperture size is comparable to the wavelength of the wave. If size >> λ, diffraction is negligible (ray optics); if size

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Question #628

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The internal energy of an ideal gas depends only on:

A. Both pressure and volume

B. Temperature

C. Volume

D. Pressure

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Correct Answer: Option B

Explanation:
For an ideal gas, internal energy U depends solely on temperature (U = f/2 nRT, where f is degrees of freedom). This follows from kinetic theory: no intermolecular forces, so energy is purely kinetic, proportional to T. Pressure and volume can change at constant T (isothermal process) without changing U. Memory tip: 'Ideal gas: U = U(T) only; real gases have volume dependence'. This thermodynamics concept is frequently tested in competitive exams. Always distinguish ideal vs real gas behavior; competitive exams typically assume ideal gas unless specified.

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Question #629

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The time taken by light to travel from Sun to Earth is approximately:

A. 8 seconds

B. 1 day

C. 1 hour

D. 8 minutes

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Correct Answer: Option D

Explanation:
Average Earth-Sun distance (1 AU) ≈ 1.5×10¹¹ m. Speed of light c = 3×10⁸ m/s. Time t = distance/speed = 1.5×10¹¹ / 3×10⁸ = 500 seconds ≈ 8.33 minutes ≈ 8 minutes. This is a standard astronomical fact. Memory aid: 'Sunlight takes 8 minutes to reach Earth; we see the Sun as it was 8 minutes ago'. This factual knowledge question tests awareness of scale in physics, frequently appearing in competitive exams. Always recall key constants: Earth-Sun distance, light speed, for quick estimations.

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Question #630

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In a series LCR circuit at resonance, the impedance is:

A. Infinite

B. Zero

C. Maximum

D. Minimum

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Correct Answer: Option D

Explanation:
At resonance frequency ω₀ = 1/√(LC), inductive reactance X_L = ωL equals capacitive reactance X_C = 1/(ωC), so they cancel. Impedance Z = √[R² + (X_L - X_C)²] = R, which is minimum (since R is the only remaining component). Current is maximum at resonance. Memory tip: 'LCR resonance: Z_min = R, I_max, purely resistive'. This AC circuit concept is frequently tested in competitive exams. Always distinguish series resonance (Z min) from parallel resonance (Z max) behavior.

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Question #631

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The specific charge (e/m) of an electron was first measured by:

A. Bohr

B. Rutherford

C. Millikan

D. Thomson

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Correct Answer: Option D

Explanation:
J. J. Thomson measured the specific charge (e/m) of electrons in 1897 using cathode ray tube experiments with electric and magnetic fields. Millikan later measured the charge e (oil drop experiment), allowing mass calculation. Rutherford discovered nucleus; Bohr developed atomic model. Memory aid: 'Thomson = e/m; Millikan = e'. This history-based question tests knowledge of experimental physics milestones, frequently appearing in competitive exams. Always link scientists to their specific measurements: Thomson (e/m), Millikan (e), Chadwick (neutron).

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Question #632

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The angle of minimum deviation for a prism depends on:

A. Angle of prism only

B. Wavelength of light only

C. Refractive index only

D. Both angle of prism and refractive index

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Correct Answer: Option D

Explanation:
Minimum deviation δ_m for a prism: n = sin[(A + δ_m)/2] / sin(A/2), where A is prism angle, n refractive index. Thus δ_m depends on both A and n. Since n varies with wavelength (dispersion), δ_m also depends on wavelength, but the primary dependencies are A and n. Memory tip: 'δ_m increases with A and n; used to measure n'. This optics formula application is frequently tested in competitive exams. Always recall that minimum deviation occurs when ray passes symmetrically through prism, a key condition for derivation.

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Question #633

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The work done in moving a charge q along an equipotential surface is:

A. qV

B. Zero

C. Depends on path

D. Infinite

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Correct Answer: Option B

Explanation:
Equipotential surface has constant potential V everywhere. Work done W = qΔV; since ΔV = 0 along equipotential, W = 0. This holds for any path on the surface. Electric field is perpendicular to equipotential surfaces, so no work is done moving charge along them. Memory aid: 'Equipotential: ΔV=0 ⇒ W=0; field lines ⊥ equipotentials'. This electrostatics concept is frequently tested in competitive exams. Always link work to potential difference, not absolute potential; path independence is key for conservative fields.

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Question #634

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The radius of the nth Bohr orbit in hydrogen atom is proportional to:

A. 1/n

B. 1/n²

C. n

D. n²

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Correct Answer: Option D

Explanation:
Bohr model: radius r_n = (4πε₀ħ²n²)/(m_e e²) = n² a₀, where a₀ is Bohr radius (≈0.529 Å). Thus r_n ∝ n². Energy E_n ∝ -1/n². Memory tip: 'Bohr radius: r ∝ n²; energy: E ∝ -1/n²'. This atomic physics formula is frequently tested in competitive exams. Always recall that n is principal quantum number; higher n means larger orbit, less tightly bound electron. This problem assesses understanding of quantization in early quantum theory.

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Question #635

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The phenomenon of polarization of light demonstrates that light is a:

A. Transverse wave

B. Matter wave

C. Longitudinal wave

D. Mechanical wave

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Correct Answer: Option A

Explanation:
Polarization (restriction of electric field oscillation to one plane) is possible only for transverse waves, where oscillations are perpendicular to propagation direction. Longitudinal waves (e.g., sound) cannot be polarized as oscillations are along propagation. This was key evidence for light's transverse wave nature. Memory aid: 'Polarization ⇒ transverse wave; light is EM transverse wave'. This conceptual question tests wave optics fundamentals, frequently examined in competitive exams. Always distinguish wave types: transverse (light, string waves) vs longitudinal (sound, spring waves).

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Question #636

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The efficiency of a machine is defined as:

A. Both (a) and (c)

B. Power output / Power input

C. Work output / Work input

D. Work input / Work output

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Correct Answer: Option A

Explanation:
Efficiency η = useful work output / total work input = useful power output / total power input, since power = work/time. Both definitions are equivalent for steady operation. Efficiency is always < 1 (or

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Question #637

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The magnetic field at the center of a circular coil of radius R carrying current I is:

A. μ₀I/(2R)

B. μ₀NI/(4πR)

C. μ₀I/(4R)

D. μ₀NI/(2R)

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Correct Answer: Option D

Explanation:
For a circular coil with N turns, magnetic field at center B = μ₀NI/(2R). For single turn (N=1), B = μ₀I/(2R). This derives from Biot-Savart law. Option A is for single turn; B and D have incorrect constants. Memory tip: 'Coil center: B = μ₀NI/(2R); straight wire: B = μ₀I/(2πr)'. This formula application is frequently tested in magnetism sections of competitive exams. Always note number of turns N; competitive exams often include it to test attention to detail.

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Question #638

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The process of heat transfer by actual movement of fluid particles is called:

A. Conduction

B. Convection

C. Advection

D. Radiation

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Correct Answer: Option B

Explanation:
Convection involves heat transfer by bulk movement of fluid (liquid or gas), carrying thermal energy. Natural convection: density differences drive flow; forced convection: external means (fan, pump). Conduction: molecular collisions without bulk motion; radiation: EM waves. Memory aid: 'Convection = fluid motion carries heat; e.g., boiling water, weather systems'. This definition question tests thermodynamics fundamentals, frequently examined in competitive exams. Always distinguish convection (requires fluid) from conduction (solids/fluids) and radiation (no medium needed).

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Question #639

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The number of significant figures in 0.00250 is:

A. 2

B. 6

C. 4

D. 3

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Correct Answer: Option D

Explanation:
Significant figures rules: leading zeros are not significant; trailing zeros after decimal are significant. Thus 0.00250: leading zeros (before 2) not significant; digits 2,5,0 are significant (trailing zero after decimal counts). So three significant figures. Memory tip: 'Start counting at first non-zero digit; include trailing zeros after decimal'. This measurement precision concept is frequently tested in competitive exams. Always apply rules consistently: 0.0025 has two sig figs; 0.00250 has three, indicating higher precision.

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Question #640

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A body is in equilibrium under three concurrent forces. If two forces are perpendicular, the third force must be:

A. Equal to vector sum of the two

B. Zero

C. Equal to sum of the two

D. Equal and opposite to vector sum of the two

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Correct Answer: Option D

Explanation:
For equilibrium, net force = 0. If forces F₁ and F₂ are perpendicular, their vector sum R = √(F₁² + F₂²). The third force F₃ must balance R, so F₃ = -R, i.e., equal in magnitude and opposite in direction to the vector sum of F₁ and F₂. Memory aid: 'Equilibrium: vector sum of all forces = 0; third force closes the triangle'. This statics concept is frequently tested in competitive exams. Always use vector addition for force equilibrium; scalar sum is incorrect for non-collinear forces.

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