Electromagnetic Waves — Class 12 MCQs with Answers

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

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Speed of EM waves in vacuum:

   A3 × 10⁸ m/sB3 × 10⁶ m/sC10⁸ m/sD10¹⁰ m/s
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   Answer: 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. Q2 of 6Easy

   The wavelength range of visible light is approximately:

   A700 nm–1 mmB1–10 nmC400–700 nmD10 cm–1 m
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   Answer: 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. Q3 of 6Medium

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

   A60 TB2.0 × 10⁻⁵ TC2.0 × 10⁻⁹ TD2.0 × 10⁻⁷ T
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   Answer: 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.

4. Q4 of 6Medium

   Assertion (A): TV broadcasts use frequencies from about 54 MHz to 890 MHz, which lie in the radio-wave region of the spectrum. Reason (R): Gamma rays are used in medicine to destroy cancer cells.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true but R is not the correct explanation of A

   Explanation: A is true: TV waves range from 54 MHz to 890 MHz, within the radio band of about 500 kHz to 1000 MHz. R is true as well, but it is about a medical use of gamma rays and says nothing about which band TV signals occupy.

5. Q5 of 6Hard

   EM waves were predicted by:

   AEinsteinBNewtonCMaxwellDHertz only
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   Answer: 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.

6. Q6 of 6Hard

   Assertion (A): Ultraviolet radiation can be focussed into very narrow beams for precision work such as LASIK eye surgery. Reason (R): Ultraviolet lamps are used to kill germs in water purifiers.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true but R is not the correct explanation of A

   Explanation: A is true: because of its short wavelength, UV can be focussed into very narrow beams. R is true as well, but germ-killing in purifiers is a different use of UV and has nothing to do with how finely it can be focussed.

Electromagnetic Waves notes: key concepts (Class 12 Physics)

Maxwell's completion of electromagnetism and its consequence: displacement current and the corrected Ampere law, the prediction that light is an electromagnetic wave, the properties that follow — transverse character, mutual perpendicularity of E and B, and the energy and momentum a wave carries — and the full electromagnetic spectrum with the uses of each band.

Displacement current

Maxwell's insight that a changing electric field acts as a current, resolving the inconsistency of Ampere's law in the gap of a charging capacitor.

Ampere-Maxwell law

Adds displacement current to conduction current in Ampere's law, making the set of electromagnetic equations complete and self-consistent.

Prediction of EM waves

Maxwell's equations predict self-sustaining waves travelling at 1/√(μ₀ε₀), a value matching the measured speed of light — establishing light as an EM wave.

Nature of EM waves

Oscillating electric and magnetic fields, mutually perpendicular and both perpendicular to the direction of propagation. No medium is needed.

Transverse character

Because E and B are perpendicular to the direction of travel, EM waves can be POLARISED — something impossible for a longitudinal wave like sound.

Speed of EM waves

In vacuum, 3 × 10⁸ m s⁻¹ for every wavelength. In a medium the speed drops to c/n, where n is the refractive index.

Ratio of E and B amplitudes

The two field amplitudes are locked together with E₀/B₀ = c, and the two fields oscillate exactly in phase.

Energy carried by an EM wave

Shared equally between the electric and magnetic fields. Intensity is proportional to the SQUARE of the electric field amplitude.

Momentum and radiation pressure

EM waves carry momentum U/c, so they exert pressure on a surface — the physics behind solar sails and comet tails.

Radio waves

The longest wavelengths, produced by accelerating charges in aerials. Used for radio, television and mobile communication.

Microwaves

Short-wavelength radio waves used in radar, satellite links and microwave ovens, where they resonate with water molecules to heat food.

Infrared radiation

Also called heat waves, emitted by all warm bodies. Used in remote controls, thermal imaging and night vision, and trapped by greenhouse gases.

Visible light

The narrow band from roughly 400 nm to 700 nm that the human eye detects — a tiny slice of the whole spectrum.

Ultraviolet radiation

Beyond violet, energetic enough to damage living tissue. The ozone layer absorbs most of the Sun's UV, which is why its depletion matters.

X-rays

Produced when fast electrons are suddenly stopped by a metal target. Used in medical imaging and crystallography for their high penetrating power.

Gamma rays

The shortest wavelength and highest energy, emitted by radioactive nuclei. Used in cancer therapy and in sterilising medical equipment.