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The speed of light, in any medium,which is usually denoted by \displaystyle{ c }, is a physical constant important in many areas of physics.It is denoted by 'c^0' especially in vacuum medium, although the symbol 'c' can be used to refer to that in any medium. It is exactly 299,792,458 metres per second (983,571,056 feet per second) by definition. A photon (particle of light) travels at this speed in a vacuum.