Speed Of Electromagnetic Waves

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A student derives the speed of light in vacuum from fundamental constants and needs the correct combination of permittivity and permeability.

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According to NCERT Class 12, Chapter 8 (Electromagnetic Waves), Maxwell's equations predict that electromagnetic disturbances travel through vacuum at a speed determined solely by the electric and magnetic properties of free space. The speed is expressed as c = 1 / √(μ₀ε₀), where μ₀ is the permeability and ε₀ is the permittivity of free space. Substituting μ₀ = 4π × 10⁻⁷ T·m/A and ε₀ = 8.85 × 10⁻¹² C²/N·m² yields approximately 3 × 10⁸ m/s, matching the measured speed of light — a triumph that showed light itself is an electromagnetic wave. The option c = μ₀ε₀ is dimensionally the inverse of speed squared, so it cannot represent a velocity. The form c = 1/(μ₀ε₀) has units of speed squared rather than speed, making it incorrect. The expression c = √(μ₀ε₀) gives the reciprocal of speed and is therefore dimensionally wrong. A dimensional check confirms that only 1/√(μ₀ε₀) yields metres per second, validating the correct choice.

Speed Of Electromagnetic Waves

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