How fast do electromagnetic waves travel in a vacuum?

• A. 299,792,458 m/s
• B. 30,000 m/s
• C. 3,000,000 m/s
• D. 300,000,000 m/s

Answer: (D)

Electromagnetic waves in a vacuum travel at the speed of light, a universal constant. Without any medium to slow them, all such waves move at the same speed, c. The exact value is 299,792,458 meters per second, defined by how we set the meter. In everyday use, it’s common to round this to about 3.00 × 10^8 m/s or 300,000,000 m/s for simple calculations. The main idea is that in empty space the speed is constant and the same for all electromagnetic waves; in other media, their speed would be slower depending on that medium’s refractive index.