Definition · Plain-language
Speed of light
The speed of light in a vacuum is a fundamental constant of nature, exactly 299,792,458 metres per second, and is the same for every observer.
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A fixed constant of nature
The speed of light in a vacuum, denoted c, is one of the most important constants in physics. Its value is fixed by definition at exactly 299,792,458 metres per second. Remarkably, this speed is the same for every observer, whether they are standing still or moving fast toward or away from the light source — a fact that lies at the heart of Einstein’s special relativity. Nothing carrying matter, energy or information can travel faster than c, so it acts as the ultimate speed limit of the universe.
Why it defines the metre
Because the speed of light is so constant and can be measured with extraordinary precision, scientists turned the relationship around. Since 1983 the metre has been defined as the distance light travels in a vacuum in 1/299,792,458 of a second. This means the speed of light is no longer measured but fixed by definition, and the metre is derived from it together with the definition of the second. Tying our units to an unchanging constant of nature makes them far more stable and reproducible than physical reference objects ever were.
Light slows in materials
The fixed value of c applies only to a vacuum. When light passes through a transparent material such as water, glass or air, it travels more slowly because it interacts with the atoms along the way. This slowing is what bends light as it enters glass or water, the effect called refraction that makes a straw look broken in a glass and makes lenses work. The ratio of the speed of light in a vacuum to its speed in a material is the material’s refractive index.
Key facts
At a glance
- Symbol: c
- Value: exactly 299,792,458 metres per second in a vacuum
- Approximate: about 300,000 km/s, or 186,000 miles per second
- Same for all: identical for every observer, whatever their motion
- Cosmic limit: the maximum speed for matter, energy or information
- Defines the metre: fixed since 1983 to define the SI unit of length
Common misconceptions
What people often get wrong
Often heard: Light always travels at the same speed everywhere.
Actually: Light travels at its fixed maximum speed only in a vacuum. In water, glass or air it moves more slowly, which is what causes refraction and lets lenses focus light.
Often heard: A fast-moving observer can catch up to a beam of light.
Actually: No observer can. The speed of light is the same for everyone regardless of their own motion, a cornerstone of special relativity, so light always recedes at c no matter how fast you chase it.
Often heard: The value of the speed of light is only approximate.
Actually: Since 1983 the speed of light has had an exact, defined value, and the metre is built from it. It is no longer a measured quantity with uncertainty but a fixed constant by definition.
Going deeper
