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How fast does light travel in a vacuum?

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A horizontal gradient bar displays the visible light spectrum from violet to red, labeled with corresponding frequency in terahertz, wavelength in nanometers, and photon energy in electron volts. — light When white light is spread apart by a prism or a diffraction grating, the colours of the visible spectrum appear. The colours vary according to their wavelengths. Violet has the highest frequencies and shortest wavelengths, and red has the lowest frequencies and the longest wavelengths.

In a vacuum, the speed of light is defined as exactly 299,792,458 meters per second.1

This value is considered a fundamental constant of nature and serves as the universal upper limit for the speed at which signals and material particles can travel. In special relativity, this constant—represented by the symbol c—is essential for linking mass and energy in the equation $E = m c^{2}$ . Furthermore, the constancy of the speed of light for all observers is a foundational principle that requires space and time to be relative rather than absolute.12