Electromagnetic Waves

part 1

Displacement Current

• Need for displacement current
• Maxwell's equations (qualitative)
• Electromagnetic waves — origin and nature

part 2

Properties of EM Waves

• Characteristics of electromagnetic waves
• Transverse nature of EM waves
• Speed of EM waves

part 3

EM Spectrum

• Electromagnetic spectrum
• Radio waves, microwaves, infrared
• Visible light, UV, X-rays, gamma rays
• Applications of each type

Speed of EM waves in vacuum

$c = \frac{1}{\sqrt{\mu_0\varepsilon_0}} = 3\times10^8$ m/s

★ frequently asked

E and B amplitudes

$\frac{E_0}{B_0} = c$

★ frequently asked

Displacement current

$I_d = \varepsilon_0 \frac{d\Phi_E}{dt}$

★ frequently asked

Generalised Ampere's law

$\oint\vec{B}\cdot d\vec{l} = \mu_0(I_c + I_d)$

★ frequently asked

Write any three characteristics of electromagnetic waves. How are the directions of the electric field, magnetic field and propagation related in an electromagnetic wave?

Write the expression for the speed of electromagnetic waves in vacuum in terms of the permeability μ₀ and permittivity ε₀ of free space.

Arrange the following electromagnetic waves in ascending order of their frequencies: microwaves, gamma rays, infrared waves, X-rays.

Name the electromagnetic waves used for (i) treatment of cancer and (ii) sterilising surgical instruments. Write one property of each.

Name the electromagnetic radiation used for (i) studying the crystal structure of solids and (ii) remote sensing. Write one source of each.

Show that the average energy density of an electromagnetic wave is equally shared by the electric and magnetic fields. Write the expression for the total average energy density.

What is displacement current? Explain how the concept of displacement current resolves the inconsistency in Ampere's circuital law, and write the Ampere–Maxwell law.

Which part of the electromagnetic spectrum has the longest wavelength, and which is used in RADAR systems?

How are electromagnetic waves produced? Why can a stationary charge or a charge in uniform motion not radiate electromagnetic waves?

How many marks is Electromagnetic Waves worth in the Class 12 board exam?

Around 4 marks of the 70-mark CBSE Physics theory paper, based on the official unit-wise weightage. Memorise the EM spectrum order — guaranteed marks.

Is Electromagnetic Waves easy or hard?

It's rated easy — one of the easier chapters in Class 12 Physics. Most students need about 2 hours to cover it well.

What are the important topics in Electromagnetic Waves?

The chapter covers 8 NCERT topics in 3 areas: Displacement Current; Properties of EM Waves; EM Spectrum.

What questions come from Electromagnetic Waves in board exams?

Between 2018–2024, CBSE board papers asked questions from this chapter on Nature of EM Waves, Speed of EM Waves, EM Spectrum, Uses of EM Waves — 1- to 3-mark questions.