Class 12 Physics Chapter 8: Electromagnetic Waves

CBSE 2026–27 | NCERT-aligned free notes, diagrams, examples and practice

Electromagnetic waves arise from changing electric and magnetic fields and can propagate through vacuum. This chapter introduces displacement current, Maxwell’s insight, wave properties and the electromagnetic spectrum.

1. Displacement Current

A changing electric field between capacitor plates produces an effect analogous to current in the circuit. Maxwell introduced displacement current to make the current concept consistent with charge conservation and electromagnetic theory.

2. Nature of EM Waves

Electromagnetic waves are transverse. Electric field E and magnetic field B are mutually perpendicular and both are perpendicular to the direction of propagation.

3. Speed

In vacuum, electromagnetic waves travel at c ≈ 3×10⁸ m/s. The wave relation is c = 1/√(μ₀ε₀).

4. Electromagnetic Spectrum

Across the spectrum, frequency increases from radio waves toward gamma rays, while wavelength decreases.

5. Important Properties

• No material medium is required for propagation through vacuum.
• They carry energy and momentum.
• They travel at c in vacuum.
• They exhibit reflection, refraction, interference, diffraction and polarisation.

Worked Example

For radiation of frequency 6×10¹⁴ Hz, λ = c/f = 3×10⁸/(6×10¹⁴) = 5×10⁻⁷ m, within the visible range.

Common Exam Traps

• EM waves do not need air or another material medium.
• Frequency does not change when light crosses into another medium; speed and wavelength can change.
• Do not reverse the order of the electromagnetic spectrum.

Practice Questions

1. Why did Maxwell introduce displacement current?
2. Explain the transverse nature of electromagnetic waves.
3. State the speed of EM waves in vacuum and its relation to μ₀ and ε₀.
4. Arrange the EM spectrum in increasing frequency.
5. Give two uses each of microwaves, infrared, ultraviolet and X-rays.

📌 Concept Diagram: Electromagnetic Spectrum

Think With Examples

• Remote control: commonly uses infrared radiation.
• Radar: uses microwaves because they can be directed and detected over long distances.
• Medical imaging: X-rays interact strongly enough with tissues of different densities to form diagnostic images.
• UV: has higher frequency and energy than visible light and can cause photochemical effects.

Application Questions

1. Why can radio waves travel through vacuum but sound cannot?
2. Which region has the shortest wavelength and which has the highest frequency?
3. Why does visible light occupy only a small part of the electromagnetic spectrum?
4. For a wave of wavelength 3 m in vacuum, calculate its frequency.

Electromagnetic Spectrum Visual

All electromagnetic waves travel at the speed of light in vacuum. They differ primarily in frequency and wavelength, related by c=νλ.

Production and Uses

Worked Concept

If an electromagnetic wave has frequency 6×10¹⁴ Hz in vacuum, λ=c/ν=(3×10⁸)/(6×10¹⁴)=5×10⁻⁷ m.

Common Mistakes

• Confusing frequency and wavelength order across the spectrum.
• Forgetting that electromagnetic waves do not require a material medium.
• Using incorrect units in c=νλ.

Practice

1. Arrange waves by wavelength/frequency.
2. Calculate wavelength from frequency.
3. Match spectrum regions with applications.
4. Explain why electromagnetic waves can propagate through vacuum.

Worked Numerical — Electromagnetic Wave

An electromagnetic wave has wavelength 600 nm. Its frequency is ν=c/λ=(3×10⁸)/(600×10⁻⁹)=5×10¹⁴ Hz.