Electromagnetic Waves Class 12 Notes | CBSE Physics Chapter 8

Electromagnetic Waves is Chapter 8 of CBSE Class 12 Physics. This chapter covers the nature of electromagnetic waves, Maxwell’s equations (qualitative), the electromagnetic spectrum, and the properties and uses of different types of EM waves — from radio waves to gamma rays.

This chapter carries 3–5 marks. The EM spectrum, properties of different waves, and displacement current concept are commonly tested.

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Key Concepts

1. Displacement Current and Maxwell’s Equations

Maxwell showed that a changing electric field produces a magnetic field, just as a changing magnetic field produces an electric field (Faraday’s law). He introduced the concept of displacement current:

Id = ε₀(dΦE/dt)

This completed Ampere’s law: ∮B·dl = μ₀(Ic + Id), making the equations symmetric.

2. Properties of EM Waves

• Produced by accelerating charges
• Do not need a medium — travel through vacuum at speed c = 3 × 10⁸ m/s
• E and B fields are perpendicular to each other and to the direction of propagation (transverse waves)
• c = 1/√(μ₀ε₀) = E₀/B₀
• Carry energy and momentum
• Follow c = fλ

3. Electromagnetic Spectrum

Wave Type	Frequency Range	Wavelength Range	Source	Uses
Radio waves	< 10⁹ Hz	> 0.3 m	Oscillating circuits	Radio/TV broadcasting, communication
Microwaves	10⁹ – 10¹¹ Hz	0.3 m – 1 mm	Klystron, magnetron	Microwave oven, radar, satellite communication
Infrared	10¹¹ – 4×10¹⁴ Hz	1 mm – 700 nm	Hot bodies	Night vision, remote controls, greenhouse effect
Visible light	4×10¹⁴ – 8×10¹⁴ Hz	700 – 400 nm	Sun, lamps	Vision, photosynthesis
Ultraviolet	8×10¹⁴ – 10¹⁶ Hz	400 – 1 nm	Sun, mercury lamp	Sterilisation, LASIK, vitamin D production
X-rays	10¹⁶ – 10¹⁹ Hz	1 nm – 10⁻³ nm	X-ray tube (bombarding metal with electrons)	Medical imaging, security screening
Gamma rays	> 10¹⁹ Hz	< 10⁻³ nm	Radioactive decay, nuclear reactions	Cancer treatment, sterilisation

Order of increasing frequency: Radio < Microwave < IR < Visible < UV < X-ray < Gamma

Order of increasing wavelength: Gamma < X-ray < UV < Visible < IR < Microwave < Radio

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Solved Examples

Example 1

An EM wave has frequency 5 × 10¹⁴ Hz. Find its wavelength and identify the type.

Answer: λ = c/f = (3 × 10⁸)/(5 × 10¹⁴) = 6 × 10⁻⁷ m = 600 nm. This is visible light (orange-red).

Example 2

The electric field in an EM wave has amplitude 100 V/m. Find the amplitude of the magnetic field.

Answer: B₀ = E₀/c = 100/(3 × 10⁸) = 3.33 × 10⁻⁷ T

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Important Questions for Board Exams

1-Mark

1. What is displacement current?
2. Which EM wave is used in RADAR?

3-Mark

1. List the electromagnetic spectrum in order of increasing frequency. Give one use of each type.
2. State the properties of electromagnetic waves.

5-Mark

1. Describe the electromagnetic spectrum in detail. Give the source, frequency range, and applications for each type of EM wave.

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Quick Revision Points

• EM waves: E ⊥ B ⊥ direction of propagation; travel at c = 3 × 10⁸ m/s in vacuum
• c = 1/√(μ₀ε₀) = E₀/B₀ = fλ
• Displacement current Id = ε₀(dΦE/dt) — completes Ampere’s law
• Spectrum (↑ frequency): Radio → Microwave → IR → Visible → UV → X-ray → Gamma
• Radio: broadcasting; Microwave: radar, ovens; IR: night vision; UV: sterilisation; X-ray: imaging; Gamma: cancer treatment

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Previous: Ch 7 — Alternating Current
Next: Ch 9 — Ray Optics