Speed of light

The speed of light in vacuum is commonly denoted as c. This is a universal physical constant. 

The speed of light is 186,000 miles per second or 300,000 kilometers per second.

According to special relativity, c is the upper limit for the speed that conventional matter and information can travel at in the physical universe. This speed is most commonly associated with light, however, it is also the speed at which gravitational waves and electromagnetic radiation propagate at. 

In both special and general relativity, the speed of light finds its way into the equations, interrelating space and time. c also, appears in the famous mass-energy equivalence equation: e=mc^2. 

Can light or other electromagnetic radiation travel at a speed slower than c? Light, actually, while propagating through transparent materials, will have a speed less than c. Also, the speed of electromagnetic waves in wire cables is shown to be slower than c. 

In 1676, Ole Romer, demonstrated that light travels at a finite speed. Light was not instantaneous. Romer arrived at this conclusion by examining Io, the moon of Jupiter.

In 1865, James Clerk Maxwell, proposed that light was an electromagnetic wave. Thus, light would travel at the speed of c, a term that appeared in Maxwell's equation's for electromagnetism. 

In 1905, with Albert Einstein's special theory of relativity, c, is a constant with respect to any reference frame. This constant would also be independent of motion of the light source.