The speed of light in a vacuum is accepted as a universal constant, the same value for all observers and all types of electromagnetic radiation. First determined in 1676 by Danish scientist Olaf Rømer using astronomical observations, it is now expressed in electromagnetic quantities through Maxwell's equations. The speed of light in a vacuum is defined to be exactly 299,792,458 meters per second (about 186,000 miles per second), thus fixing the length of the meter in terms of the second. The speed of light can be altered in certain conditions, such as by gravitational, magnetic, or material interference.
Symbol: c
See also:
special relativity