Speed of light

The speed of light (usually denoted c) is a physical constant. It is the fastest speed at which energy or information can travel, and is only attained by massless particles and waves such as electromagnetic radiation (e.g. radio waves, visible light, or gamma rays) in vacuum, where there are no atoms, molecules or other types of matter that can slow it down. Its value is exactly 299,792,458 metres per second, often approximated as 300,000 kilometres per second or 186,000 miles per second (see the table on the right for values in other units).
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