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Electromagnetic Waves – Class 12 Physics notes, important questions and MCQs

NCERT Class 12 Physics, chapter 8. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Accelerating charges produce electromagnetic waves — oscillating electric and magnetic fields travelling at the speed of light. They form the electromagnetic spectrum.

Properties

E and B are perpendicular to each other and to the direction of travel (transverse).
Speed in vacuum c = 1/√(µ₀ε₀) = 3 × 10⁸ m/s; c = E₀/B₀.
Maxwell added displacement current, I_d = ε₀ dΦ_E/dt.

Spectrum (increasing frequency)

Radio → microwaves → infrared → visible → ultraviolet → X-rays → gamma rays.
Uses: radio (communication), microwaves (radar, ovens), IR (remote controls, night vision), UV (sterilisation), X-rays (medical imaging), gamma (cancer treatment).

Key terms

Displacement current: current due to changing electric field
Electromagnetic spectrum: range of EM waves ordered by wavelength or frequency
Transverse nature of EM waves: E and B oscillate perpendicular to each other and to the direction of travel

Formulas

c = 1/√(µ₀ε₀)
c = fλ
E₀/B₀ = c

Important questions with answers

What is displacement current and why did Maxwell introduce it?2 marks

I_d = ε₀ dΦ_E/dt, a current-like term due to a changing electric field. Maxwell added it so that Ampere's law stays consistent, for example between the plates of a charging capacitor where there is no conduction current.

Give one use each of microwaves, ultraviolet rays and X-rays.3 marks

Microwaves: radar and microwave ovens. Ultraviolet: sterilising water and surgical instruments (also detecting fake notes). X-rays: medical imaging of bones and checking luggage.

List five properties of electromagnetic waves and calculate the wavelength of a 100 MHz FM radio wave and the frequency of light of wavelength 500 nm.5 marks

EM waves need no medium; they travel at c = 3 × 10⁸ m/s in vacuum (c = 1/√(µ₀ε₀)); they are transverse with E ⟂ B ⟂ direction of propagation; E and B are in phase with E₀ = cB₀; they carry energy and momentum and can exert radiation pressure. λ = c/f = 3 × 10⁸/10⁸ = 3 m. f = c/λ = 3 × 10⁸/(5 × 10⁻⁷) = 6 × 10¹⁴ Hz.

Quick revision

EM waves are transverse.
Gamma rays have the highest frequency.
Radio waves have the longest wavelength.

Exam tips

All EM waves travel at the same speed in vacuum; they differ in frequency and wavelength.
Order by increasing frequency: radio, microwave, infrared, visible, ultraviolet, X-rays, gamma rays.

Practice MCQs

Speed of EM waves in vacuum is
1. 3 × 10⁶ m/s
2. 3 × 10⁸ m/s
3. 3 × 10¹⁰ m/s
4. 340 m/s
Show answer
Answer: (b) 3 × 10⁸ m/s. Speed of light.
Which has the highest frequency?
1. radio waves
2. gamma rays
3. X-rays
4. visible light
Show answer
Answer: (b) gamma rays. Gamma.
Waves used in TV remote controls are
1. ultraviolet
2. infrared
3. X-rays
4. microwaves
Show answer
Answer: (b) infrared. IR.
The concept of displacement current was given by
1. Faraday
2. Hertz
3. Maxwell
4. Ampere
Show answer
Answer: (c) Maxwell. Maxwell.
The ratio E₀/B₀ for an EM wave in vacuum equals
1. 1
2. 1/c
3. c
4. µ₀ε₀
Show answer
Answer: (c) c. E = cB.
In an EM wave, if E₀ = 300 V/m, then B₀ is
1. 10⁻⁶ T
2. 10⁻⁸ T
3. 9 × 10¹⁰ T
4. 3 × 10⁻⁶ T
Show answer
Answer: (a) 10⁻⁶ T. 300/(3 × 10⁸).
Which radiation is used to kill germs in water purifiers?
1. ultraviolet
2. infrared
3. radio
4. microwave
Show answer
Answer: (a) ultraviolet. UV sterilisation.
Which EM waves are used in remote controls of TVs
1. microwaves
2. infrared
3. ultraviolet
4. X-rays
Show answer
Answer: (b) infrared. IR LEDs.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.