Electromagnetic Waves – JEE Mains Physics
1. Introduction
Electromagnetic waves (EM waves) are waves that are created as a result of vibrations between an electric field and a magnetic field. They carry energy and can travel through the vacuum of space.
2. Displacement Current
Maxwell introduced the concept of displacement current to modify Ampere’s Law for non-steady currents. It is given by: Id = ε₀ (dΦE/dt) , where ΦE is the electric flux.
3. Maxwell’s Equations (in vacuum)
- Gauss’s Law for Electricity:
∇·E = ρ/ε₀ - Gauss’s Law for Magnetism:
∇·B = 0 - Faraday’s Law of Induction:
∇×E = -dB/dt - Ampère-Maxwell Law:
∇×B = μ₀J + μ₀ε₀(dE/dt)
4. Properties of Electromagnetic Waves
- Transverse in nature
- Do not require a medium
- Travel at the speed of light in vacuum:
c = 1/√(μ₀ε₀) - Electric and magnetic fields are perpendicular to each other and to the direction of wave propagation
5. Electromagnetic Spectrum
The full range of EM waves is categorized based on frequency or wavelength:
- Radio Waves: Used in communication
- Microwaves: Radar, cooking (microwave ovens)
- Infrared (IR): Night vision, remote controls
- Visible Light: ROYGBIV, only part we can see
- Ultraviolet (UV): Sterilization, Vitamin D synthesis
- X-rays: Medical imaging
- Gamma Rays: Produced in nuclear reactions, highly penetrating
6. Energy and Momentum of EM Waves
EM waves carry both energy and momentum. Energy density is given by: u = ε₀E² = B²/μ₀
Momentum carried is p = E/c
7. Applications of Electromagnetic Waves
- Communication: Radio, TV, mobile networks
- Medical: MRI, X-ray, radiation therapy
- Scientific: Spectroscopy, astronomy