83 practice questions on Electromagnetic Waves , sorted Easy → Hard. Try each one first, then open its answer page for the worked explanation. Want the full theory first? Read the Electromagnetic Waves notes .
Electromagnetic Wave: E ⊥ B ⊥ Direction of Travel propagation E (electric field, vertical plane) B (magnetic field, horizontal plane, perpendicular to E) In an electromagnetic wave, the oscillating electric field (E) and magnetic field (B) are perpendicular to each other and to the direction the wave travels - a purely transverse wave that needs no medium, unlike sound.
Easy - 27 questions Q1.
A microwave has a wavelength of 3 cm in vacuum. Its frequency is:
A 1 × 10<sup>10</sup> HzB 1 × 10<sup>9</sup> HzC 1 × 10<sup>11</sup> HzD 3 × 10<sup>8</sup> HzShow answer & explanation →
Q2.
The speed of electromagnetic waves in free space is given by:
A √(μ<sub>0</sub>ε<sub>0</sub>)B 1/√(μ<sub>0</sub>ε<sub>0</sub>)C μ<sub>0</sub>ε<sub>0</sub>D 1/(μ<sub>0</sub>ε<sub>0</sub>)Show answer & explanation →
Q3.
In an electromagnetic wave, the directions of E, B and propagation are:
A All parallelB Mutually perpendicularC E and B parallel, wave perpendicularD RandomShow answer & explanation →
Q5.
Electromagnetic waves were first experimentally produced and detected by:
A HertzB MaxwellC FaradayD OerstedShow answer & explanation →
Q7.
Electromagnetic waves transport:
A Both energy and momentumB Electric charge onlyC Rest mass onlyD Neither of theseShow answer & explanation →
Q8.
Electromagnetic waves are produced by:
A Oscillating or accelerating electric chargesB Stationary charges sitting at rest in a fixed electric fieldC Constant, unchanging magnetic fields with no time variationD Moving uncharged particles such as neutrons or neutrinosShow answer & explanation →
Q9.
The speed of electromagnetic waves in vacuum is:
A 3 × 10⁸ m/s (same for all EM waves)B 3 × 10⁸ m/s mainly for light in most casesC Depends on the frequency under typical conditionsD Depends on the wavelength according to standard textbooksShow answer & explanation →
Q10.
In an electromagnetic wave, the electric field E and magnetic field B are:
A Perpendicular to each other and to the direction of propagationB Parallel to each other in general practice as frequently describedC Parallel to the direction of propagation in most textbook accountsD E is parallel while B is perpendicular to propagation during normal conditionsShow answer & explanation →
Q11.
Which part of the electromagnetic spectrum has the highest frequency?
A Gamma raysB X-raysC UltravioletD Radio wavesShow answer & explanation →
Q12.
Microwaves are used in cooking because they:
A Cause water molecules to vibrate (dielectric heating)B Are strongly absorbed and reflected by metal cookwareC Have wavelengths longer than AM radio broadcast wavesD Cause ionization of atoms within the food, like X-raysShow answer & explanation →
Q13.
X-rays are used in medical imaging because they:
A Penetrate soft tissue but are absorbed by denser materials like boneB Carry no ionizing energy and pose no risk to living tissue as generally observedC Are largely blocked by skin and cannot enter the body in typical laboratory settingsD Reflect specularly off bone surfaces like light off a mirror under usual circumstancesShow answer & explanation →
Q14.
Gamma rays are produced by:
A Nuclear transitions (radioactive decay)B Ordinary chemical reactions such as combustionC Electrons accelerating and decelerating in a metal targetD Thermal emission from objects at everyday temperaturesShow answer & explanation →
Q15.
Infrared radiation is associated with:
A Heat (thermal radiation from warm objects)B High-energy nuclear reactions according to most researchersC Radio communications in the majority of cases studiedD Ionizing radiation as widely reportedShow answer & explanation →
Q16.
The displacement current was introduced by Maxwell because:
A Ampere's law was inconsistent for a changing electric field (capacitor plates)B Ordinary conduction current was too small in magnitude to detect in standard practiceC Magnetic monopoles had just been experimentally discovered under most conditions encounteredD Electric current was found to flow freely through a vacuum gap as frequently observed in practiceShow answer & explanation →
Q17.
The relationship between electric and magnetic field amplitudes in an EM wave is:
A E<sub>0</sub>/B<sub>0</sub> = c (speed of light)B E<sub>0</sub> = B<sub>0</sub>, with both fields numerically equalC E<sub>0</sub> = cB_0², with the magnetic amplitude squaredD E<sub>0</sub>/B<sub>0</sub> = 1/c, the inverse of the actual relationShow answer & explanation →
Q18.
Ultraviolet radiation is responsible for:
A Sunburn and skin cancer (damages DNA), also synthesizes vitamin DB Synthesizing vitamin D in skin with little other biological effectC Passing through skin largely with little measurable biological effectD Producing a tan with little deeper cellular or DNA-level damageShow answer & explanation →
Q19.
Radio waves are used for communication because they:
A Travel long distances, can be reflected by the ionosphere, and carry modulated informationB Travel at exactly twice the speed of light in vacuum under all conditions in many documented casesC Pass through every obstacle in their path with zero attenuation or scattering according to conventional understandingD Are directly visible to the unaided naked human eye like ordinary sunlight in routine practiceShow answer & explanation →
Q20.
The ozone layer in the atmosphere protects Earth by absorbing:
A Ultraviolet radiationB Infrared radiationC Radio wavesD Visible lightShow answer & explanation →
Q22.
Which property does an electromagnetic wave NOT possess?
A It requires a medium to propagateB It carries energy and momentumC It can be polarizedD It travels at speed c in vacuumShow answer & explanation →
Q23.
The frequency of visible light (wavelength 500 nm) is approximately:
A 6 × 10¹⁴ HzB 6 × 10⁸ HzC 5 × 10⁶ HzD 3 × 10⁸ HzShow answer & explanation →
Q24.
The intensity of an EM wave is proportional to:
A The square of the amplitude (E<sub>0</sub>² or B<sub>0</sub>²)B The amplitude E<sub>0</sub> raised to the first power onlyC The frequency of oscillation of the waveD The wavelength of the propagating waveShow answer & explanation →
Q25.
Maxwell unified electricity, magnetism, and optics by showing that:
A Light is an electromagnetic waveB Magnetic monopoles existC Electric charge is quantizedD All forces are equal in strengthShow answer & explanation →
Q26.
RADAR (Radio Detection and Ranging) uses:
A Microwaves (cm-wavelength)B AM radio waves overallC Gamma rays in most casesD Infrared under typical conditionsShow answer & explanation →
Medium - 24 questions Q28.
In a plane electromagnetic wave travelling in vacuum, the ratio of the energy density stored in the electric field to that in the magnetic field is:
A 1B cC 1/cD c<sup>2</sup>Show answer & explanation →
Q29.
A radio wave has a frequency of 6 × 10<sup>7</sup> Hz. Its wavelength in vacuum is:
A 50 mB 0.5 mC 5 mD 500 mShow answer & explanation →
Q30.
A parallel plate capacitor is being charged. The displacement current between its plates is:
A Equal to the conduction currentB Zero at every instant of timeC Half the conduction current valueD Twice the conduction current valueShow answer & explanation →
Q31.
The wavelengths of gamma rays, visible light and radio waves are in the order:
A gamma > visible > radioB gamma < visible < radioC visible < gamma < radioD radio < visible < gammaShow answer & explanation →
Q32.
A microwave oven operates at 2.45 GHz. What is the wavelength of the microwaves used?
A 12.2 cmB 2.45 cmC 1 mD 3 mmShow answer & explanation →
Q33.
The Poynting vector S gives:
A Energy flux (power per unit area) of an EM wave, S = E × B/μ_0B The force exerted on a single point charge by the fieldC The electric field energy density alone, ignoring the magnetic partD The magnetic field energy density alone, ignoring the electric partShow answer & explanation →
Q34.
Radiation pressure of an EM wave on a perfectly absorbing surface is:
A P = I/c (intensity divided by speed of light)B P = Ic, multiplying intensity by the speed of light insteadC P = I/c², dividing by the square of the speed of lightD P = 2I/c, the value for a perfectly reflecting surface insteadShow answer & explanation →
Q35.
In which region of the EM spectrum does VIBGYOR (visible light) lie?
A Between infrared and ultraviolet (wavelengths 400-700 nm)B Between radio waves and microwaves in the long-wavelength regionC Between ultraviolet and X-rays in the short-wavelength regionD Between X-rays and gamma rays at the highest-energy endShow answer & explanation →
Q36.
The energy density of an EM wave has equal contributions from E and B fields:
A u = ε_0 E² = B²/μ_0 (each term equal at any instant)B u = ε_0 E² mainly, ignoring the magnetic field's contribution largelyC u = B²/μ_0 mainly, ignoring the electric field's contribution largelyD u = E² + B², adding the fields without their respective constantsShow answer & explanation →
Q37.
Which EM radiation is used in PET (Positron Emission Tomography) scans?
A Gamma rays (from positron-electron annihilation)B X-rays generated by an external X-ray tube sourceC Radio waves emitted by excited atomic nucleiD Microwaves absorbed selectively by body tissueShow answer & explanation →
Q38.
The ionosphere reflects radio waves of frequency below a critical frequency. This is used in:
A AM radio broadcasting (long distance, ground and sky waves)B FM radio, which instead relies on direct line-of-sight propagationC Optical fiber communication, which uses total internal reflection in glassD Satellite TV, which relies on direct transmission through the ionosphereShow answer & explanation →
Q39.
Displacement current in a capacitor charging is:
A I<sub>d</sub> = ε_0 dΦ_E/dt (linked to changing electric flux)B Equal to the conduction current in the wire in routine practiceC Zero usually overall in most cases under typical conditionsD The current through the dielectric according to standard textbooksShow answer & explanation →
Q40.
Which characteristic of an EM wave changes when it passes from vacuum to glass?
A Frequency remains same; wavelength and speed decreaseB Speed stays same; frequency and wavelength changeC All three changeD Only frequency changesShow answer & explanation →
Q41.
The wavelength of gamma rays is typically:
A Less than 0.01 nm (< 10⁻² nm)B 1-10 nm, typical of soft X-rays insteadC 100-400 nm, typical of ultraviolet light insteadD 0.01-10 m, typical of radio waves insteadShow answer & explanation →
Q42.
EM waves carry linear momentum. The momentum of a photon of frequency f is:
A p = hf/c = h/λB p = hfC p = hf²/cD p = hc/fShow answer & explanation →
Q43.
Infrared waves are used in:
A Night vision cameras, TV remotes, and detecting heat sourcesB Mainly long-range radio broadcast communications as widely reportedC Medical X-ray imaging of bones and dense tissue in standard practiceD Mainly visible-light photography applications under most conditions encounteredShow answer & explanation →
Q44.
According to Maxwell, a changing magnetic field creates:
A An electric field (Faraday-Maxwell law)B Another magnetic field in general practiceC A gravitational field as frequently describedD A current in nearby wires mainly in most textbook accountsShow answer & explanation →
Q45.
If the electric field amplitude of an EM wave is E<sub>0</sub> = 100 V/m, what is the magnetic field amplitude?
A B<sub>0</sub> = 3.33 × 10⁻⁷ TB B<sub>0</sub> = 100 TC B<sub>0</sub> = 3 × 10¹⁰ TD B<sub>0</sub> = 100/c TShow answer & explanation →
Q46.
Long wavelength radio waves can diffract around mountains and follow Earth's curvature because:
A Their wavelength is comparable to or larger than obstacle dimensionsB They travel through air at a higher speed than short radio wavesC They carry more photon energy than shorter-wavelength radio wavesD They are always vertically polarized unlike shorter radio wavesShow answer & explanation →
Q47.
The greenhouse effect works because:
A CO<sub>2</sub> and H<sub>2</sub>O absorb outgoing infrared radiation from Earth but allow incoming visible lightB CO<sub>2</sub> blocks essentially all incoming sunlight before it reaches the surface during normal conditionsC Ozone in the upper atmosphere absorbs outgoing infrared radiation as generally observedD Clouds uniformly block all incoming and outgoing radiation largely in typical laboratory settingsShow answer & explanation →
Q48.
Frequency modulation (FM) radio is better quality than amplitude modulation (AM) because:
A Noise creates amplitude variations (easily filtered in FM); FM has wider bandwidth for more audio detailB FM signals generally travel a much farther physical distance through the atmosphere than AM signalsC FM uses inherently shorter wavelengths that carry noticeably more photon energy per cycle under usual circumstancesD AM signals are far more strongly affected by the Doppler shift than FM signals are according to most researchersShow answer & explanation →
Q49.
The electric field of an EM wave is E = E<sub>0</sub> sin(kx - ωt). The wave is traveling in the:
A Positive x directionB Negative x directionC y directionD z directionShow answer & explanation →
Q50.
The intensity of electromagnetic radiation from a point source varies with distance r as:
A I ∝ 1/r² (inverse square law)B I ∝ 1/r, falling off linearly rather than quadraticallyC I ∝ r², growing instead of shrinking with distanceD I stays exactly constant regardless of distance from the sourceShow answer & explanation →
Q51.
In the EM spectrum, as frequency increases:
A Wavelength decreases and photon energy increasesB Wavelength increases and energy increasesC Wavelength decreases and energy decreasesD All stay constantShow answer & explanation →
Hard - 32 questions Q52.
A plane electromagnetic wave has a peak electric field of 48 V/m. Its average intensity (S = ½ε<sub>0</sub>cE<sub>0</sub><sup>2</sup>) is approximately:
A 6.1 W/m<sup>2</sup>B 1.5 W/m<sup>2</sup>C 12.2 W/m<sup>2</sup>D 3.1 W/m<sup>2</sup>Show answer & explanation →
Q53.
For the same sunlight of intensity 1.4 × 10<sup>3</sup> W/m<sup>2</sup> on a perfectly reflecting surface, the radiation pressure is:
A 2.3 × 10<sup>-6</sup> PaB 4.7 × 10<sup>-6</sup> PaC 1.4 × 10<sup>-5</sup> PaD 9.3 × 10<sup>-6</sup> PaShow answer & explanation →
Q54.
A point source radiates 100 W of electromagnetic power uniformly. The intensity at 2 m is:
A 3.98 W/m<sup>2</sup>B 1.99 W/m<sup>2</sup>C 7.96 W/m<sup>2</sup>D 0.99 W/m<sup>2</sup>Show answer & explanation →
Q55.
An EM wave carrying energy 6 × 10<sup>-8</sup> J is completely absorbed by a surface. The momentum transferred is:
A 6 × 10<sup>-16</sup> kg m/sB 1 × 10<sup>-16</sup> kg m/sC 2 × 10<sup>-16</sup> kg m/sD 2 × 10<sup>-15</sup> kg m/sShow answer & explanation →
Q56.
Displacement current was introduced by Maxwell to account for:
A Ordinary conduction current flowing through resistorsB Current through capacitor (changing E field)C Current flowing through an ideal inductor coilD Steady DC current flowing through a simple closed loopShow answer & explanation →
Q57.
In electromagnetic waves, E and B fields are:
A Parallel to each other and to propagation direction according to conventional understandingB Perpendicular to each other and both perpendicular to propagationC Parallel to each other, perpendicular to propagation in routine practiceD Antiparallel to propagation overall in most cases under typical conditionsShow answer & explanation →
Q58.
Derive the speed of light from Maxwell's equations: c = 1/√(μ_0ε_0). Given μ_0 = 4π×10⁻⁷ H/m and ε_0 = 8.85×10⁻¹² F/m. Calculate c.
A 3 × 10⁸ m/sB 1 × 10⁸ m/sC 6 × 10⁸ m/sD 1.5 × 10⁸ m/sShow answer & explanation →
Q60.
An EM wave has electric field E = 6 sin(ωt - kx) V/m. The average intensity is:
A 0.0477 W/m²B 0.096 W/m²C 6 W/m²D 36 W/m²Show answer & explanation →
Q61.
A laser beam (λ = 600 nm, power = 1 mW, beam area = 1 mm²) exerts radiation pressure on a perfect mirror. The force is:
A 6.67 × 10⁻⁹ NB 6.67 × 10⁻⁶ NC 3.33 × 10⁻⁹ ND 1 × 10⁻³ NShow answer & explanation →
Q62.
The wave equation for the electric field component of an EM wave is:
A ∇²E = μ_0ε_0 ∂²E/∂t² (wave equation with speed c)B ∇²E = -kE, the time-independent Helmholtz equation formC ∇E = μ_0ε_0 ∂E/∂t, using a first derivative instead of second derivativesD ∇E = 0, as if the electric field had no spatial variation at allShow answer & explanation →
Q63.
In Compton scattering, an X-ray photon scatters off an electron. The wavelength shift is:
A Δλ = (h/m<sub>e</sub> c)(1 - cosθ) (Compton formula)B Δλ = hν, dimensionally an energy rather than a wavelength shiftC Δλ = 0 (no change), as in purely elastic Thomson scatteringD Δλ = hf/c, dimensionally a momentum rather than a wavelength shiftShow answer & explanation →
Q64.
The skin depth (depth at which EM wave amplitude falls to 1/e) in a conductor is:
A δ = √(2/μσω) where σ is conductivityB δ = c/f, the plain vacuum wavelength formula with no conductivity termC δ = λ/(2π), the formula for the radian wavelength in vacuumD δ = λ, equating the skin depth directly to the free-space wavelengthShow answer & explanation →
Q66.
The critical frequency (MUF - Maximum Usable Frequency) for ionospheric reflection depends on:
A Electron density in the ionosphere (f<sub>c</sub> = 9√N where N = electron density)B The absolute temperature of the ionospheric layer aloneC Earth's magnetic field strength alone, with no electron density termD The physical height of the ionospheric layer aloneShow answer & explanation →
Q67.
Synchrotron radiation is emitted when:
A Relativistic electrons travel in circular paths in magnetic fieldsB Electrons undergo direct head-on collisions with protons in routine practiceC A heavy nucleus undergoes spontaneous nuclear fission overallD Ordinary thermal blackbody emission from a hot plasma in most casesShow answer & explanation →
Q68.
If the amplitude of an EM wave in vacuum is E<sub>0</sub>, in a medium of refractive index n (transparent), the E field amplitude:
A √(n) E<sub>0</sub> (slightly higher due to slower phase velocity)B E<sub>0</sub>/nC E<sub>0</sub> nD E<sub>0</sub>/√nShow answer & explanation →
Q69.
The Cherenkov radiation is produced when:
A A charged particle moves faster than light in that mediumB An atom undergoes ordinary radioactive nuclear decayC Two photons collide and annihilate each other directlyD A charged particle decelerates suddenly, as in X-ray productionShow answer & explanation →
Q70.
In the Sun, energy reaches Earth as visible light but is re-emitted as infrared. The peak wavelength (λ_max) for a blackbody at 5800 K (Sun) by Wien's law is:
A 500 nm (visible)B 10000 nm (IR)C 100 nm (UV)D 1 mm (microwave)Show answer & explanation →
Q71.
Which phenomenon demonstrates the vector nature of the electric field in EM waves?
A Polarization (wave can be polarized only for transverse waves with vector displacement)B Ordinary reflection of the wave off a flat boundary surface under typical conditionsC Ordinary refraction of the wave passing into a denser medium according to standard textbooksD The Doppler effect shifting the wave's observed frequency in general practice as frequently describedShow answer & explanation →
Q72.
The average energy density of an EM wave is related to the electric field by:
A <u> = ε_0 E<sub>0</sub>²/2 = ε_0 E<sub>rms</sub>²B <u> = ε_0 E<sub>0</sub>² in most textbook accountsC <u> = ε_0 E<sub>0</sub>²/4 during normal conditionsD <u> = 2ε_0 E<sub>0</sub>² as generally observedShow answer & explanation →
Q73.
In medical MRI (Magnetic Resonance Imaging), the EM waves used are:
A Radio waves (RF pulses, ~64 MHz for 1.5 T MRI)B X-rays generated by a high-voltage electron tubeC Gamma rays emitted from a radioactive tracer isotopeD Microwaves used for dielectric heating of tissueShow answer & explanation →
Q74.
The concept of displacement current ensures conservation of charge (continuity equation). In a capacitor being charged with current I, the displacement current between plates is:
A Exactly equal to I (continuous current through circuit)B Zero, as if no current flowed between the capacitor platesC I/2, mainly half of the conduction current charging the capacitorD 2I, twice the conduction current charging the capacitorShow answer & explanation →
Q76.
The speed of electromagnetic waves in vacuum, 1/√(μ₀ε₀), is:
A 1×10⁸ m/sB 3×10⁶ m/sC 3×10⁸ m/sD 9×10⁸ m/sShow answer & explanation →
Q77.
In an electromagnetic wave in vacuum, the ratio of the amplitudes of the electric field to the magnetic field (E₀/B₀) equals:
Show answer & explanation →
Q78.
Among radio waves, X-rays, ultraviolet, and gamma rays, the one with the highest frequency is:
A radio wavesB ultravioletC X-raysD gamma raysShow answer & explanation →
Q79.
An electromagnetic wave carries energy 3×10⁸ J and is completely absorbed. The momentum delivered is:
A 3×10⁻⁸ kg·m/sB 1 kg·m/sC 3×10⁸ kg·m/sD 9×10¹⁶ kg·m/sShow answer & explanation →
Q80.
The concept added by Maxwell to Ampere law to make it consistent is the:
A displacement currentB conduction currentC eddy currentD drift currentShow answer & explanation →
Q82.
The average intensity of an electromagnetic wave in vacuum in terms of the peak electric field E₀ is:
A (1/2)cε₀E₀²B cε₀E₀²C (1/2)ε₀E₀²D cE₀²Show answer & explanation →
Q83.
The type of electromagnetic radiation commonly used in TV remote controls is:
A infraredB ultravioletC X-raysD gamma raysShow answer & explanation →