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OpenDisplacement current, properties of EM waves and the electromagnetic spectrum.
Displacement current, properties of EM waves and the electromagnetic spectrum. Short and mostly theory-based.
Physics · Class 12 · Electromagnetic Waves
Electromagnetic Waves for JEE
Everything you need for this topic on one page: the ideas, the formulas, the traps, and free lectures that explain it well.
Last updated: 26 October 2026
Key ideas
Electromagnetic Waves: what you must understand
- EM waves are transverse, with E and B perpendicular to each other and to the direction of travel
- They need no medium
- Energy is shared equally between electric and magnetic fields
- Order of the spectrum by wavelength: radio > micro > IR > visible > UV > X-ray > gamma
Formula sheet
Must-know formulas for Electromagnetic Waves
- c = 1/√(μ₀ε₀) = E₀/B₀
- Displacement current I_d = ε₀ dΦ_E/dt
- Intensity I = ½ε₀E₀²c
- Radiation pressure = I/c (absorbed)
Print
One-page formula sheet
Printable Electromagnetic Waves formula sheet: formulas, key ideas and traps on one page. Save it as a PDF for offline revision.
Avoid
Common mistakes students make
- Mixing up the order of the spectrum
- Forgetting the factor ½ in average intensity
- Using E₀ = B₀ without c
For the exam
How this topic is asked in JEE
Appears regularly; quick marks. Usually one easy theory question. Memorise the spectrum with uses.
Prerequisites: Electromagnetic Induction.
NCERT map
Where Electromagnetic Waves is in NCERT
Chapter names follow the current rationalised NCERT textbooks. Read these sections before solving PYQs.
| Class | Book | Chapter | Sections JEE asks from |
|---|---|---|---|
| Class 12 | Physics Part I | Electromagnetic Waves | Displacement Current; Electromagnetic Waves; Electromagnetic Spectrum; Sources of Electromagnetic Waves; Nature of Electromagnetic Waves |
JEE Advanced level
A JEE Advanced-level example on Electromagnetic Waves
Question. A parallel plate capacitor with circular plates of radius R = 10 cm is being charged. The electric field between the plates increases uniformly at a rate dE/dt = 3 × 10^13 V/(m·s). Find (a) the displacement current between the plates, (b) the magnetic field at a distance r = 5 cm from the central axis, and (c) for an EM wave, what is the ratio of magnetic energy density to electric energy density?
Show solution
- Displacement current: Id = ε₀ × A × (dE/dt) = ε₀ × πR² × (dE/dt) = (8.85 × 10⁻¹²) × π × (0.1)² × (3 × 10¹³) = 8.34 A
- For magnetic field at r < R, use Ampere-Maxwell law. Current enclosed by radius r: Id(enc) = ε₀ × (dE/dt) × πr²
- Apply ∮B·dl = μ₀ × Id(enc), giving B × 2πr = μ₀ε₀ × (dE/dt) × πr², so B = (μ₀ε₀/2) × r × (dE/dt)
- Using μ₀ε₀ = 1/c² = 1/(9 × 10¹⁶): B = (0.05 × 3 × 10¹³)/(2 × 9 × 10¹⁶) = 8.3 × 10⁻⁶ T = 8.3 μT
- For EM waves: uB = B²/(2μ₀) and uE = ε₀E²/2. Since E = cB for EM waves, the ratio uB/uE = B²/(μ₀ε₀E²) = B²/(μ₀ε₀c²B²) = 1/(μ₀ε₀c²) = 1
Answer: (a) Id = 8.34 A, (b) B = 8.3 μT, (c) Ratio = 1 (equal energy densities in EM waves)
This example was solved twice independently and both solutions agree.
Free lectures
Best free lectures for Electromagnetic Waves
Full-chapter free lectures picked from YouTube search results by views, from established JEE channels (mostly in Hinglish). Use the subtopic list to jump straight to the part you need (timestamps come from each video's own chapter list). Watch one, then solve PYQs the same day.
- ELECTROMAGNETIC WAVES in One Shot: All Concepts & PYQs Covered |JEE Main & Advanced — JEE Wallah, 2:57:35
- Electromagnetic Waves ONE SHOT | Class 12th Physics | JEE Mains & Advance — Lakshya JEE, 1:18:33
Practise
Where to practise for free
- 8 JEE Main 2026 PYQs on Electromagnetic Waves, with solutions.
- Read the chapter in NCERT textbooks (official, free), then solve its NCERT Exemplar (official, free) questions.
- Chapter-wise past questions: JEE Main and Advanced PYQs and question bank.
- Timed practice tests: NTA Abhyas practice tests (official, free) and our free mock tests.
- See it move: interactive simulation.
FAQs
Frequently asked questions
Is Electromagnetic Waves important for JEE Main 2027?
Appears regularly; quick marks. It is part of the official 'Electromagnetic Waves' unit in the JEE Main 2027 syllabus.
What are the most important formulas in Electromagnetic Waves?
c = 1/√(μ₀ε₀) = E₀/B₀; Displacement current I_d = ε₀ dΦ_E/dt; Intensity I = ½ε₀E₀²c.
How should I study Electromagnetic Waves for JEE?
Understand the key ideas from NCERT or one full lecture, learn the formula sheet, then solve 25–30 past JEE questions on the topic. Usually one easy theory question. Memorise the spectrum with uses.