JEE Main Physics Electromagnetic Waves 2027: Displacement Current & the EM Spectrum
Displacement current, the nature of EM waves, E and B relations, and the electromagnetic spectrum with uses, in one quick, high-clarity guide.
Edurack
September 28, 2026
This is the shortest chapter in the Physics syllabus, and also one of the most predictable. Electromagnetic Waves usually contribute a single quick question, so treat it as a chapter you can finish in one focused evening.
A changing electric field creates a magnetic field, and Maxwell's insight turned that into light.
Chapter at a Glance
| Snapshot | Detail |
|---|---|
| NTA unit | Unit 15 of 20 — Electromagnetic Waves |
| Priority (trend-based) | Low-moderate |
| Typical question style | Short conceptual MCQs on the spectrum and wave properties |
| Best first step | Learn the spectrum order and the E/B = c relation |
Priority reflects past-paper trends, not an official NTA weightage.
What the NTA Syllabus Covers
- Displacement current
- Electromagnetic waves and their characteristics, transverse nature
- Electromagnetic spectrum: radio waves, microwaves, infrared, visible, ultraviolet, X-rays, gamma rays
- Applications of electromagnetic waves
Master These Topics
1. Displacement Current: Maxwell's Fix
While a capacitor charges, current flows in the wires but no conduction current crosses the gap between the plates. Ampere's law seemed to break. Maxwell resolved this by adding a displacement current:
I_d = ε₀ dΦ_E / dt
A changing electric flux acts like a current in producing a magnetic field. Inside the capacitor gap the displacement current equals the conduction current in the wires.
2. Properties of EM Waves
- They are transverse: E, B and the direction of propagation are mutually perpendicular.
- In vacuum they travel at
c = 1/√(μ₀ε₀) ≈ 3 × 10⁸ m/s. - Amplitudes are related by
E₀ / B₀ = c. - Energy is shared equally between electric and magnetic fields.
Worked example: If E₀ = 30 V/m, then B₀ = 30 / (3 × 10⁸) = 10⁻⁷ T, or 100 nT. A wave of frequency 3 × 10⁹ Hz has wavelength λ = c/f = 0.1 m.
Trap: EM waves need no medium and carry momentum and energy. They are not deflected by electric or magnetic fields, since photons are uncharged.
3. The EM Spectrum in Order
From lowest to highest frequency (approximate wavelength ranges):
| Region | Approximate wavelength | Typical use |
|---|---|---|
| Radio waves | above 0.1 m | Broadcasting, communication |
| Microwaves | 1 mm to 0.1 m | Microwave ovens, radar |
| Infrared | 700 nm to 1 mm | Remote controls, night vision |
| Visible | 400 nm to 700 nm | Vision |
| Ultraviolet | 1 nm to 400 nm | Sterilisation, sun tanning |
| X-rays | 10⁻³ nm to 1 nm | Medical imaging |
| Gamma rays | below 10⁻³ nm | Cancer treatment, sterilising equipment |
Memory hook: frequency and energy increase as you go down the table, while wavelength decreases.
Common Traps to Avoid
- Thinking EM waves are longitudinal. They are transverse.
- Assuming they need a medium to travel.
- Swapping the order of ultraviolet and infrared in the spectrum.
- Forgetting that displacement current exists only where the electric flux changes.
60-Second Revision Sheet
I_d = ε₀ dΦ_E/dtc = 1/√(μ₀ε₀),E₀/B₀ = c- Order: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma
- Transverse, no medium needed, carry energy and momentum
Your Study Plan
- Day 1: displacement current logic and Maxwell's equations at concept level.
- Day 2: spectrum order, uses and a set of past-paper conceptual questions.
Practice Electromagnetic Waves Questions Free → (opens in a new tab)
Continue Your Physics Journey
- Previous chapter: Electromagnetic Induction & Alternating Currents
- Next chapter: Optics
- All 20 JEE Main Physics chapters
- Complete JEE Main Syllabus 2027 guide
Frequently Asked Questions
Why is displacement current needed?
Without it, Ampere's law fails for a charging capacitor, because no conduction current passes through the gap. Displacement current restores consistency.
What is the relation between E and B in an EM wave?
In vacuum the amplitudes satisfy E₀ / B₀ = c, and E and B are always in phase.