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[[Image:lighterstill.jpg]]
[[Image:lighterstill.jpg]]
[[Image:PrismAndLight.jpg|right|thumb|300px|A beam of white light (entering upwards from the right) is dispersed into its constituent colors by its passage through a [[triangular prism (optics)|prism]]. The fainter beam of white light exiting to the upper right has been reflected (without dispersion) off the first surface of the prism.]]
[[Image:PrismAndLight.jpg|right|frame]].
'''Light''', or '''visible light''', is [[electromagnetic radiation]] of a [[wavelength]] that is visible to the human [[eye]] (about 400–700 [[nanometre|nm]]). In a [[Science|scientific]] context, the word ''light'' is sometimes used to refer to the entire [[electromagnetic spectrum]].[http://www.lightsources.org/cms/?pid=1000166] (What Is a Light Source?) Light is composed of an [[elementary particle]] called a [[photon]].
'''Light''', or '''visible light''', is [[electromagnetic radiation]] of a [[wavelength]] that is visible to the human [[eye]] (about 400–700 [[nanometre|nm]]). In a [[Science|scientific]] context, the word ''light'' is sometimes used to refer to the entire [[electromagnetic spectrum]].[http://www.lightsources.org/cms/?pid=1000166] (What Is a Light Source?) Light is composed of an [[elementary particle]] called a [[photon]].
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==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.
[[Image:Speed of light from Earth to Moon.gif|thumb|center|460px|A line showing the speed of light on a scale model of [[Earth]] and the [[moon]], about 1.2 seconds.]]
[[Image:Speed of light from Earth to Moon.gif|thumb|center|460px|A line showing the speed of light on a scale model of [[Earth]] and the [[moon]], about 1.2 seconds.]]
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.