What happens to the speed of the electromagnetic energy traveling through air as the altitude increases?

• A. Signal speed decreases
• B. Signal speed increases
• C. Signal speed remains the same
• D. Speed becomes variable

Answer: (B)

Electromagnetic waves move through a medium at a speed determined by that medium’s refractive index. Air has a refractive index just above 1, so waves travel slightly slower than in a vacuum. As altitude increases, the air becomes thinner and its refractive index drops toward 1, causing the wave speed to rise toward the speed of light in vacuum. Therefore, the signal speed increases with altitude. The change is small because air remains only a little denser than vacuum even at low altitudes, but the trend is clear: faster as you go higher.