Electromagnetic Waves — Class 12 MCQs with Answers

Electromagnetic Waves — important questions & MCQs with answers (Class 12 Physics)

14 solved questions from this chapter's difficulty levels, each with its answer and explanation. The other 76 are timed and scored when you sign in.

1. Q1Easy

   Speed of EM waves in vacuum:

   A.3 × 10⁸ m/s✓ Correct

   B.3 × 10⁶ m/s

   C.10⁸ m/s

   D.10¹⁰ m/s

   Answer: A. 3 × 10⁸ m/s

   Explanation: Maxwell's equations give the speed of light in vacuum as c = 1/√(μ₀ε₀) ≈ 3×10⁸ m/s — a fixed constant for ALL EM waves, not just visible light. Picking a rounder but wrong power of ten is the usual slip.

2. Q2Easy

   The wavelength range of visible light is approximately:

   A.700 nm–1 mm

   B.1–10 nm

   C.400–700 nm✓ Correct

   D.10 cm–1 m

   Answer: C. 400–700 nm

   Explanation: Visible light spans roughly 400 nm (violet) to 700 nm (red); wavelengths outside this range — UV below 400 nm, IR above 700 nm — aren't detected by the human eye.

3. Q3Easy

   EM waves are:

   A.Sound waves

   B.Longitudinal

   C.Mechanical

   D.Transverse✓ Correct

   Answer: D. Transverse

   Explanation: EM waves are transverse: E and B oscillate perpendicular to each other and to the direction of travel, unlike sound which is longitudinal and needs a medium — that's the usual mix-up.

4. Q4Easy

   Which EM waves are also called heat waves?

   A.Ultraviolet

   B.Infrared✓ Correct

   C.X-rays

   D.Radio waves

   Answer: B. Infrared

   Explanation: Infrared is called heat radiation because water molecules in skin and tissue absorb IR strongly and convert it to thermal energy — unlike UV, which is higher-frequency but poorly absorbed as heat.

5. Q5Easy

   Do electromagnetic waves require a material medium to propagate?

   A.Yes, always

   B.No, they can travel through vacuum✓ Correct

   C.Only through metals

   D.Only through gases

   Answer: B. No, they can travel through vacuum

   Explanation: EM waves are self-sustaining oscillations of E and B that regenerate each other via Faraday's and Maxwell-Ampere's laws, so unlike sound they need no medium and travel through vacuum at c.

6. Q6Easy

   Harmful solar ultraviolet radiation is absorbed mainly by:

   A.Carbon dioxide

   B.Water vapour

   C.Ozone layer✓ Correct

   D.Nitrogen

   Answer: C. Ozone layer

   Explanation: The stratospheric ozone layer absorbs harmful UV-B and UV-C before they reach the surface; CO₂ and water vapour instead trap outgoing IR (greenhouse effect), a different mechanism entirely.

7. Q7Medium

   In an EM wave the peak electric field is E₀ = 60 V/m. The peak magnetic field B₀ is approximately:

   A.60 T

   B.2.0 × 10⁻⁵ T

   C.2.0 × 10⁻⁹ T

   D.2.0 × 10⁻⁷ T✓ Correct

   Answer: D. 2.0 × 10⁻⁷ T

   Explanation: The wave relation gives B₀ = E₀/c = 60/(3×10⁸) = 2.0×10⁻⁷ T; forgetting to divide by c (giving 60 T) or misplacing a power of ten is the usual slip on this substitution.

8. Q8Medium

   Increasing frequency order:

   A.Radio < Microwave < IR < Visible < UV < X-ray < γ✓ Correct

   B.γ < X-ray

   C.Visible < Radio

   D.All same

   Answer: A. Radio < Microwave < IR < Visible < UV < X-ray < γ

   Explanation: Frequency and wavelength are inversely related, so increasing frequency runs opposite to the usual wavelength listing: radio < microwave < IR < visible < UV < X-ray < γ, ending at the shortest wavelength.

9. Q9Medium

   Average energy density of an EM wave with E_rms = 1 V/m in vacuum is approximately:

   A.8.85 × 10⁻¹⁸ J/m³

   B.8.85 × 10⁻⁶ J/m³

   C.8.85 × 10⁻¹² J/m³✓ Correct

   D.1 J/m³

   Answer: C. 8.85 × 10⁻¹² J/m³

   Explanation: Average energy density in vacuum is u = ε₀E_rms², so u = 8.85×10⁻¹² × 1² = 8.85×10⁻¹² J/m³ — dropping ε₀ entirely or using ½E₀² instead of E_rms² gives the wrong power of ten.

10. Q10Medium

    X-rays are used in:

    A.FM radio

    B.Cooking

    C.Medical imaging✓ Correct

    D.Heaters

    Answer: C. Medical imaging

    Explanation: X-rays penetrate soft tissue but are absorbed by bone, giving the sharp contrast used in medical imaging — cooking and heating instead rely on microwave or IR absorption, not X-ray penetration.

11. Q11Medium

    Intensity of an EM wave is given by:

    A.ε₀ c E₀

    B.½ ε₀ c E₀²✓ Correct

    C.½ μ₀ c E₀²

    D.c/E₀

    Answer: B. ½ ε₀ c E₀²

    Explanation: Intensity equals average energy density times speed, I = u_avg × c = ½ε₀cE₀² — using μ₀ instead of ε₀, or forgetting the ½ from time-averaging sin², are the usual slips.

12. Q12Hard

    EM waves were predicted by:

    A.Einstein

    B.Newton

    C.Maxwell✓ Correct

    D.Hertz only

    Answer: C. Maxwell

    Explanation: Maxwell mathematically predicted EM waves from his unified equations well before any experiment; Hertz later confirmed them experimentally, but the theoretical prediction itself is credited to Maxwell.

13. Q13Hard

    Typical frequency of X-rays is of order:

    A.10¹⁸ Hz✓ Correct

    B.10¹⁰ Hz

    C.10¹⁴ Hz

    D.10²⁴ Hz

    Answer: A. 10¹⁸ Hz

    Explanation: X-ray wavelengths near 0.1 nm give f = c/λ = (3×10⁸)/(10⁻¹⁰) ≈ 3×10¹⁸ Hz, order 10¹⁸ Hz — far higher than visible light's ~10¹⁴ Hz, matching X-rays' much higher photon energy.

14. Q14Hard

    Energy of EM photon:

    A.hf (or hc/λ)✓ Correct

    B.½mc²

    C.mgh

    D.qV

    Answer: A. hf (or hc/λ)

    Explanation: The Planck-Einstein relation gives photon energy as E = hf = hc/λ — proportional to frequency, not to mc² or qV, which describe rest-mass energy and electrical potential energy respectively.

Electromagnetic Waves — FAQs

More Class 12 Physics chapters

• 01Electric Charges and Fields
• 02Electrostatic Potential and Capacitance
• 03Current Electricity
• 04Moving Charges and Magnetism
• 05Magnetism and Matter
• 06Electromagnetic Induction

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