In physics, the speed of light (usually denoted * c*) is a fundamental physical constant, the speed at which light and all electromagnetic radiation travel in a perfect vacuum, which is 299,792,458 metres per second (about 300,000 kilometres per second or 186,000 miles per second). This constant is significant in the understanding and study of relativity, spacetime, astronomy, space travel, and other fields.

For much of human history the nature of light, including whether it was instantaneous or simply travelled very quickly, was unknown. By the 11th century many scientists had suggested that light had a finite speed but it was not until the 17th century that Ole RÃ¸mer demonstrated this by observing small differences in the apparent orbital period of Jupiter's moon Io. Using these observations, Christiaan Huygens estimated the speed of light to be about 220,000 km per second. Since then, scientists have devised more sophisticated techniques to improve the precision of measurement. By the mid-20th century, the speed of light was known to be approximately 299,792 kilometres per second. In 1983, the metre was redefined in the International System of Units (SI) as the distance travelled by light in vacuum in 1/299792458 of a second. As a result, *c* is fixed at exactly 299,792,458 metres per second.

According to Albert Einstein's special theory of relativity, *c* is an important constant connecting space and time in the unified structure of spacetime. As such, it defines the conversion between mass and energy and is an upper bound on the speed at which matter and information can travel. It is the speed of travel of all electromagnetic radiation in free space,and is believed to be the speed of gravitational waves.In an inertial frame of reference, light in vacuum always travels at *c*. When light passes through a transparent material, such as glass or air, its speed is reduced; the ratio between *c* and the speed at which light travels in a material is called the refractive index of the material.