The speed of light in air is approximately 299,702 km/s, or about 99.97% of its value in a vacuum (299,792 km/s).

Core Value

This slight slowdown occurs because air has a refractive index of roughly 1.000293 at standard conditions, causing light to interact minimally with air molecules. For most practical and educational purposes—like physics problems or calculations—it's often rounded to 3 × 10⁸ m/s , the same as in vacuum, since the difference is negligible (just 90 meters per second slower).

Precise Calculation

Exact speed in dry air at sea level (15°C, 1 atm):
c_air = c_vacuum / n ≈ 299,792,458 m/s ÷ 1.000293 = 299,702,547 m/s.

  • That's 186,282 miles per second in imperial units.
  • Variations depend on temperature, humidity, and pressure; warmer/moister air slows it more slightly.

Quick Comparisons

Medium| Speed (km/s)| % of Vacuum| Refractive Index
---|---|---|---
Vacuum| 299,792| 100%| 1.0000007
Air| 299,702| 99.97%| 1.0002933
Water| 225,000| 75%| 1.3339
Glass| 200,000| 67%| 1.5009

Why It Matters

Light's near-constant speed in air enables technologies like GPS (relies on precise timing) and fiber optics testing, where even tiny medium differences affect signals. Imagine racing light: in air, you'd still lag by 300,000 km in one second—highlighting why it's a cosmic speed limit!

TL;DR: ~299,702 km/s (practically 3×10⁸ m/s); tiny dip from vacuum due to air's density.

Information gathered from public forums or data available on the internet and portrayed here.