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Class 12 Physics Chapter 8: Electromagnetic Waves MCQs with answers

10 multiple-choice questions on Electromagnetic Waves, each with its answer and the reason, written from the NCERT chapter. For the CBSE board, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter.

Question 1
For a plane electromagnetic wave travelling in vacuum, the ratio of the electric field amplitude to the magnetic field amplitude, E₀/B₀, equals
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Answer: (a) the speed of light, c

In an electromagnetic wave the field amplitudes are related by B₀ = E₀/c, so E₀/B₀ = c. 1/c inverts the ratio; √(μ₀/ε₀) has the unit of resistance (ohm), not of speed; μ₀ε₀ equals 1/c².

Question 2
A plane electromagnetic wave travels in vacuum along the −y direction. At a certain point and instant its electric field points along +z. The magnetic field at that point and instant points along
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Answer: (a) −x

B is perpendicular to both E (z) and the direction of travel (y), so it lies along ±x, and E × B must point along −y. k̂ × (−î) = −ĵ, so B is along −x. With +x, E × B would point along +y; B cannot lie along the direction of travel (+y) or along E (−z).

Question 3
The displacement current through a surface across which the electric flux is Φ_E is
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Answer: (d) ε₀ dΦ_E/dt

id = ε₀ dΦ_E/dt, the rate of change of electric flux times ε₀. μ₀ε₀ dΦ_E/dt is the term as it appears in the Ampere–Maxwell law, μ₀ times the current; the magnetic flux Φ_B belongs to Faraday's law.

Question 4
The uniform electric field between the plates of a capacitor, each of area 0.020 m², is increasing at the rate of 2.0 × 10¹² V m⁻¹ s⁻¹. Taking ε₀ = 8.85 × 10⁻¹² C² N⁻¹ m⁻², the displacement current between the plates is
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Answer: (c) 0.35 A

id = ε₀ dΦ_E/dt = ε₀ A dE/dt = 8.85 × 10⁻¹² × 0.020 × 2.0 × 10¹² ≈ 0.35 A. 0.18 A halves it; 35 A slips two powers of ten; no charge crosses the gap, but a changing field carries a displacement current.

Question 5
A broadcasting station transmits on a wavelength of 3.0 m. Taking c = 3.0 × 10⁸ m s⁻¹, its frequency and the service it belongs to are
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Answer: (c) 100 MHz, FM radio

ν = c/λ = 3.0 × 10⁸/3.0 = 1.0 × 10⁸ Hz = 100 MHz, inside the FM band of 88 MHz to 108 MHz. The AM band is 530 kHz to 1710 kHz, so 100 MHz is not AM; 1.0 MHz slips a factor of 100; 900 MHz would need λ ≈ 0.33 m.

Question 6
The frequency of the microwaves in a microwave oven is chosen so that
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Answer: (c) it matches the resonant frequency of water molecules, which take up the energy as kinetic energy

The oven's frequency is selected to match the resonant frequency of water molecules, so energy goes efficiently into their kinetic energy and any food containing water heats up. Microwaves are far below visible frequencies and do not ionise molecules.

Question 7
Ritu sits all afternoon in a sunlit room behind closed glass windows, yet her skin does not tan. This is because ordinary glass
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Answer: (b) absorbs ultraviolet radiation

Tanning is the skin's response to ultraviolet: exposure induces more melanin. Ordinary glass absorbs ultraviolet, so the radiation that would tan her does not reach her; visible light clearly passes, since the room is sunlit.

Question 8
One common way of producing X-rays is to
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Answer: (a) bombard a metal target with high-energy electrons

X-rays are commonly generated by bombarding a metal target with high-energy electrons, as in an X-ray tube. Hot bodies give infrared and visible light; a semiconductor diode (an LED) gives infrared or light; radioactive nuclei emit gamma rays.

Question 9
Infrared waves are often called heat waves because they
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Answer: (d) set entire atoms and molecules vibrating, raising the internal energy of matter

Infrared, of lower frequency than visible light, vibrates not only electrons but whole atoms and molecules (water in particular absorbs it readily), raising their internal energy and temperature. All electromagnetic waves travel at the same speed in vacuum; any hot body emits infrared.

Question 10
Which arrangement is in order of decreasing wavelength?
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Answer: (b) microwaves, infrared, ultraviolet, X-rays

From Table 8.1: microwaves 0.1 m to 1 mm, infrared 1 mm to 700 nm, ultraviolet 400 nm to 1 nm, X-rays 1 nm to 10⁻³ nm. Option (a) is the increasing order; (c) swaps two pairs; (d) swaps infrared and ultraviolet.

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