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→Speed of light
==Speed of light==
==Speed of light==
The speed of light in a [[vacuum]] is exactly 299,792,458 [[Metre per second|m/s]] (about 186,282.397 miles per second). The speed of light depends upon the medium in which it is traveling, and the speed will be lower in a transparent medium. Although commonly called the "velocity of light", technically the word ''[[velocity]]'' is a [[vector (spatial)|vector]] quantity, having both magnitude and direction. ''Speed'' refers only to the magnitude of the velocity vector. This fixed definition of the speed of light is a result of the modern attempt, in physics, to define the basic unit of length in terms of the speed of light, rather than defining the speed of light in terms of a length.
The speed of light in a [[vacuum]] is exactly 299,792,458 [[Metre per second|m/s]] (about 186,282.397 miles per second). The speed of light depends upon the medium in which it is traveling, and the speed will be lower in a transparent medium. Although commonly called the "velocity of light", technically the word ''[[velocity]]'' is a [[vector (spatial)|vector]] quantity, having both magnitude and direction. ''Speed'' refers only to the magnitude of the velocity vector. This fixed definition of the speed of light is a result of the modern attempt, in physics, to define the basic unit of length in terms of the speed of light, rather than defining the speed of light in terms of a length.
Different physicists have attempted to measure the speed of light throughout history. [[Galileo Galilei|Galileo]] attempted to measure the speed of light in the seventeenth century. A good early experiment to measure the speed of light was conducted by [[Ole Rømer]], a Danish physicist, in 1676. Using a telescope, Ole observed the motions of [[Jupiter]] and one of its [[natural satellite|moon]]s, [[Io (moon)|Io]]. Noting discrepancies in the apparent period of Io's orbit, Rømer calculated that light takes about 18 minutes to traverse the diameter of Earth's orbit. Unfortunately, this was not a value that was known at that time. If Ole had known the diameter of the earth's orbit, he would have calculated a speed of 227,000,000 m/s.
Different physicists have attempted to measure the speed of light throughout history. [[Galileo Galilei|Galileo]] attempted to measure the speed of light in the seventeenth century. A good early experiment to measure the speed of light was conducted by [[Ole Rømer]], a Danish physicist, in 1676. Using a telescope, Ole observed the motions of [[Jupiter]] and one of its [[natural satellite|moon]]s, [[Io (moon)|Io]]. Noting discrepancies in the apparent period of Io's orbit, Rømer calculated that light takes about 18 minutes to traverse the diameter of Earth's orbit. Unfortunately, this was not a value that was known at that time. If Ole had known the diameter of the earth's orbit, he would have calculated a speed of 227,000,000 m/s.