Speed of electromagnetic waves in vacuum

Steps:

• Recall that electromagnetic waves include radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma rays.
• Note that all travel at the same speed in vacuum, given by c = 3 × 10^8 m/s.
• Evaluate options: A matches the universal speed; B contradicts light speed; C misplaces wavelengths; D understates smallest wavelengths.
• Confirm A as correct by electromagnetic theory.

Why A is correct:

• Maxwell's equations show all electromagnetic waves propagate at speed c = 1/√(ε₀μ₀) ≈ 3 × 10^8 m/s in vacuum.

Why the others are wrong:

• B: In air, refractive index >1 slows electromagnetic waves slightly below c, not faster.
• C: Radio waves have the largest wavelengths, far exceeding infrared.
• D: Gamma rays have the smallest wavelengths, shorter than X-rays.

Final answer: A