Chapter 15 – Electromagnetic Waves (JEE Physics)
1. Introduction to Electromagnetic Waves
Electromagnetic waves are waves that consist of oscillating electric and magnetic fields
and can propagate through vacuum without any material medium.
This is a major difference from mechanical waves, which require a medium.
2. Maxwell’s Prediction
James Clerk Maxwell unified electricity and magnetism and predicted that
a changing electric field produces a magnetic field and vice versa.
Speed of electromagnetic waves:
$$c = \frac{1}{\sqrt{\mu_0 \varepsilon_0}}$$
The calculated value of $c$ matched the speed of light, proving that light is an electromagnetic wave.
3. Production of Electromagnetic Waves
Electromagnetic waves are produced by:
- Accelerated electric charges
- Oscillating currents
A stationary charge does not produce EM waves.
4. Nature of Electromagnetic Waves
- Electromagnetic waves are transverse
- Electric field $\vec{E}$ and magnetic field $\vec{B}$ oscillate perpendicular to each other
- Both fields are perpendicular to direction of propagation
$$\vec{E} \perp \vec{B} \perp \vec{c}$$
5. Relation Between Electric and Magnetic Fields
$$E = cB$$
This relation is valid for electromagnetic waves in vacuum.
6. Energy in Electromagnetic Waves
Energy in EM waves is equally shared between electric and magnetic fields.
$$u_E = u_B$$
7. Momentum and Radiation Pressure
Electromagnetic waves carry momentum and exert pressure on surfaces.
Radiation pressure:
$$P = \frac{I}{c} \quad (\text{perfect absorber})$$
$$P = \frac{2I}{c} \quad (\text{perfect reflector})$$
8. Electromagnetic Spectrum
The electromagnetic spectrum is the complete range of electromagnetic waves arranged
according to frequency or wavelength.
| Region | Wavelength | Application |
|---|---|---|
| Radio waves | Largest | Communication |
| Microwaves | cm to mm | Radar, cooking |
| Infrared | mm to μm | Thermal imaging |
| Visible light | 400–700 nm | Vision |
| Ultraviolet | Shorter than visible | Sterilization |
| X-rays | Very short | Medical imaging |
| Gamma rays | Shortest | Cancer treatment |
9. Visible Spectrum
Visible light lies between:
- Violet: ~400 nm
- Red: ~700 nm
VIBGYOR order is important for JEE questions.
10. Speed of Electromagnetic Waves in Media
$$v = \frac{c}{\sqrt{\mu_r \varepsilon_r}}$$
Speed of EM waves decreases in material media.
11. Electromagnetic Waves Are Not Deflected by Fields
Since EM waves carry no charge, they are not deflected by electric or magnetic fields.
12. Important Characteristics of EM Waves
- Do not require medium
- Travel with speed of light in vacuum
- Carry energy and momentum
- Obey superposition principle
13. Applications of Electromagnetic Waves
- Communication (radio, TV, mobile)
- Medical imaging (X-rays)
- Satellite communication (microwaves)
- Remote sensing (infrared)
14. Common JEE Traps
- Assuming EM waves need a medium
- Mixing up wavelength order in spectrum
- Forgetting $E = cB$ relation
- Confusing radiation pressure formulas
15. Final Revision Checklist
You have mastered this chapter if you can:
- Explain Maxwell’s prediction
- Draw orientation of $\vec{E}$, $\vec{B}$ and direction of propagation
- Arrange EM spectrum correctly
- Solve radiation pressure numericals
- Identify applications of each EM wave