What is the speed of light in a vacuum?

Speed of light in vacuum (c) = 299,792,458 m/s ≈ 3 × 10⁸ m/s ≈ 300,000 km/s ≈ 186,000 mi/s. This is the universal speed limit per Einstein's special relativity — no information or matter can exceed it. Light slows in matter (water n=1.33, glass n=1.5, diamond n=2.4 — refractive index); the ratio c/v in matter is the index of refraction. Light is electromagnetic radiation, part of the electromagnetic spectrum: (1) Radio waves — longest wavelength, lowest frequency, lowest energy; (2) Microwaves; (3) Infrared (heat); (4) Visible light (the narrow band our eyes detect: 400-700 nm; red longest wavelength, violet shortest); (5) Ultraviolet; (6) X-rays; (7) Gamma rays — shortest wavelength, highest frequency, highest energy. All travel at c in vacuum. Wave equation: c = λf (speed = wavelength × frequency). Light from the Sun takes ~8 minutes to reach Earth (~150 million km). Light-year: distance light travels in 1 year (~9.46 trillion km), a unit of cosmic distance. Nearest star (Proxima Centauri): 4.24 light-years.