Physics XII - Chapter 8: Electromagnetic Waves
Solve Electromagnetic Waves Class 12 MCQs. Learn displacement current, EM spectrum, properties, and applications of radio waves to gamma rays.
Quick Revision
- Displacement Current: I_d = ε₀(dΦ_E/dt); completes circuit in changing E fields.
- Ampere-Maxwell Law: ∮B·dl = μ₀(I_c + I_d); modified Ampere's law with displacement current.
- Electromagnetic Waves: Transverse waves with E and B perpendicular to propagation.
- Wave Speed: c = 1/√(μ₀ε₀) ≈ 3 × 10⁸ m/s; speed of light in vacuum.
- E and B Relationship: E₀/B₀ = c; E and B in phase and perpendicular.
- Wave Equation: ∂²E/∂x² = μ₀ε₀(∂²E/∂t²); describes EM wave propagation.
- Electromagnetic Spectrum: Range from radio waves to gamma rays by frequency/wavelength.
- Radio Waves: λ: 1mm-100km; radio, TV, communication.
- Microwaves: λ: 1mm-1m; radar, cooking, satellite communication.
- Infrared Waves: λ: 700nm-1mm; heat radiation, remote controls.
- Visible Light: λ: 400-700nm; human vision, photography.
- Ultraviolet: λ: 10-400nm; sterilization, vitamin D production.
- X-rays: λ: 0.01-10nm; medical imaging, security screening.
- Gamma Rays: λ: 0.01nm; nuclear reactions, cancer treatment.
- Energy Density: u = ½ε₀E² + ½(B²/μ₀) = ε₀E²; energy in EM waves.
- Intensity: I = (½)ε₀cE₀²; power per unit area carried by wave.
- Momentum: p = U/c; EM waves carry momentum and exert radiation pressure.
- Poynting Vector: S = (E × B)/μ₀; direction and magnitude of energy flow.
- Sources of EM Waves: Accelerated charges, molecular vibrations, nuclear transitions.
- Atmospheric Windows: Wavelength ranges that pass through atmosphere.
- Maxwell's Equations: Complete set describing all electromagnetic phenomena.
Basic Level Questions
Chapter Summary
Electromagnetic Waves represents one of the most profound unifications in physics - the stunning realization that light, electricity, and magnetism are all manifestations of the same fundamental phenomenon. This chapter reveals how James Clerk Maxwell's brilliant insight about displacement current completed the puzzle, showing that changing electric fields can create magnetic fields just as changing magnetic fields create electric fields, leading to self-sustaining waves that travel through empty space.
We discover that these electromagnetic waves are nature's ultimate messengers, traveling at the universe's speed limit - the speed of light. The elegant relationship between electric and magnetic fields in these waves creates a perfect partnership where each sustains the other, oscillating in harmony as they journey through space. What makes this discovery even more remarkable is that visible light, which our eyes have evolved to detect, represents just a tiny sliver of the vast electromagnetic spectrum.
The electromagnetic spectrum unfolds before us like a cosmic keyboard, with each range of frequencies playing its own unique role in our universe and technology. From the long, gentle waves that carry radio broadcasts across continents to the incredibly energetic gamma rays born in nuclear furnaces of distant stars, we learn how different wavelengths interact with matter in distinct ways. Practical applications surround us - the microwaves that heat our food, the infrared that lets us see heat, the X-rays that reveal our bones, and the radio waves that connect our global civilization.
This chapter culminates in the beautiful symmetry of Maxwell's equations, which many physicists consider the most elegant and powerful set of equations ever written, governing everything from the light from distant galaxies to the wireless signals on our phones.