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Notes Of Electromagnetic Waves Class 12

By on Friday, May 23, 2025

Electromagnetic waves (EM waves) play a crucial role in modern physics and technology. They are responsible for wireless communication medical imaging and even how we perceive light. In Class 12 Physics Electromagnetic Waves is a fundamental topic covering their properties equations and applications.

This topic provides detailed notes on electromagnetic waves for Class 12 explaining their nature types Maxwell’s equations and practical uses in everyday life.

1. What Are Electromagnetic Waves?

Definition

Electromagnetic waves are transverse waves consisting of oscillating electric and magnetic fields perpendicular to each other and the direction of wave propagation. Unlike mechanical waves they do not require a medium and can travel through a vacuum.

Formation of Electromagnetic Waves

Electromagnetic waves are produced when an electric charge accelerates. This can happen in various ways such as in radio transmitters oscillating circuits and atomic transitions.

Key Properties of Electromagnetic Waves

  • Travel at the speed of light (c = 3 × 10⁸ m/s in a vacuum).
  • Do not require a medium.
  • The electric and magnetic fields are perpendicular to each other.
  • Carry energy and momentum.
  • Follow the wave equation.

2. Maxwell’s Equations and Electromagnetic Waves

James Clerk Maxwell formulated four equations that describe the behavior of electromagnetic fields and how they generate waves.

Maxwell’s Four Equations

  1. Gauss’s Law for Electricity

    • The electric flux through a closed surface is proportional to the charge enclosed.
    • Formula: ∮ E · dA = Q/ε₀

  2. Gauss’s Law for Magnetism

    • Magnetic field lines form closed loops; no isolated magnetic charges exist.
    • Formula: ∮ B · dA = 0

  3. Faraday’s Law of Electromagnetic Induction

    • A changing magnetic field induces an electric field.
    • Formula: ∮ E · dl = – dΦB / dt

  4. Ampere-Maxwell Law

    • A changing electric field generates a magnetic field.
    • Formula: ∮ B · dl = μ₀ (I + ε₀ dΦE / dt)

These equations prove that changing electric and magnetic fields sustain each other forming self-propagating electromagnetic waves.

3. Wave Equation for Electromagnetic Waves

From Maxwell’s equations the wave equation for an electromagnetic wave in a vacuum is:

frac{∂^2 E}{∂t^2} = c^2 frac{∂^2 E}{∂x^2}
frac{∂^2 B}{∂t^2} = c^2 frac{∂^2 B}{∂x^2}

This confirms that electric and magnetic fields oscillate perpendicularly to the wave’s direction and travel at the speed of light.

4. Properties of Electromagnetic Waves

1. Nature of EM Waves

  • Transverse in nature (electric and magnetic fields are perpendicular).
  • Can travel through a vacuum (unlike sound waves).
  • Travel at the speed of light (c = 3 × 10⁸ m/s).

2. Energy and Momentum

  • EM waves carry energy and momentum.

  • The energy per unit volume is given by:

    U = frac{1}{2} left( epsilon_0 E^2 + frac{B^2}{mu_0} right)

  • The momentum carried is responsible for the radiation pressure exerted by EM waves.

3. Polarization

  • Since EM waves are transverse they can be polarized (restricting vibrations to a single plane).
  • Used in sunglasses 3D movies and LCD screens.

5. Electromagnetic Spectrum

Electromagnetic waves exist in various forms classified based on their wavelength and frequency. This classification is known as the electromagnetic spectrum.

Types of Electromagnetic Waves

Type of EM Wave Frequency (Hz) Wavelength (m) Applications
Radio Waves 10³ – 10⁹ >1m Communication radio TV
Microwaves 10⁹ – 10¹² 1mm – 1m Cooking radar satellite communication
Infrared (IR) Rays 10¹² – 10¹⁵ 700nm – 1mm Remote controls thermal imaging
Visible Light 4 × 10¹⁴ – 7 × 10¹⁴ 400nm – 700nm Human vision photography
Ultraviolet (UV) Rays 10¹⁵ – 10¹⁷ 10nm – 400nm Sterilization Vitamin D synthesis
X-rays 10¹⁷ – 10¹⁹ 0.01nm – 10nm Medical imaging security scanning
Gamma Rays >10¹⁹ <0.01nm Cancer treatment nuclear reactions

Visible Light Spectrum

Visible light is the part of the EM spectrum we can see. It ranges from violet (shortest wavelength) to red (longest wavelength).

6. Applications of Electromagnetic Waves

1. Communication Systems

  • Radio waves are used in AM/FM radio and mobile communication.
  • Microwaves enable satellite and GPS technology.

2. Medical Uses

  • X-rays are used for diagnosing bone fractures.
  • Ultraviolet rays kill bacteria and viruses.

3. Industrial Applications

  • Infrared rays are used in night-vision cameras.
  • Lasers (visible light) are used in fiber optic communication.

4. Space and Astronomy

  • Telescopes detect X-rays and gamma rays from distant galaxies.
  • Infrared sensors help in studying exoplanets.

7. Mathematical Relations in Electromagnetic Waves

1. Speed of Electromagnetic Waves

The speed of light c is related to the permittivity ( epsilon_0 ) and permeability ( mu_0 ) of free space:

c = frac{1}{sqrt{epsilon_0 mu_0}}

2. Relationship Between Wavelength and Frequency

For any wave the wavelength (λ) and frequency (f) are related by:

c = flambda

Where:

  • c = speed of light
  • f = frequency
  • lambda = wavelength

8. Important Concepts for Exams

  • Maxwell’s Equations
  • Wave Equation for electric and magnetic fields
  • Electromagnetic Spectrum (order uses properties)
  • Energy and Momentum of EM Waves
  • Applications in Daily Life

Electromagnetic waves are one of the most important topics in Class 12 Physics with applications ranging from communication to medical imaging. Understanding their properties Maxwell’s equations and practical uses helps students grasp the fundamental principles of modern physics and technology.

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