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White Light

Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of any wavelength. The elementary particle that defines light is the photon. The three basic dimensions of light (i.e., all electromagnetic radiation) are: more...

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Intensity (or amplitude), which is related to the human perception of brightness of the light,; Frequency (or wavelength), perceived by humans as the colour of the light, and; Polarization (or angle of vibration), which is only weakly perceptible by humans under ordinary circumstances.;

Due to the wave-particle duality of matter, light simultaneously exhibits properties of both waves and particles. The precise nature of light is one of the key questions of modern physics.

Visible electromagnetic radiation

The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. There are no exact bounds to the visible spectrum; a typical human eye will respond to wavelengths from 400 to 700 nm, although some people may be able to perceive wavelengths from 380 to 780 nm. A light-adapted eye typically has its maximum sensitivity at around 555 nm, in the green region of the optical spectrum (see: luminosity function). The spectrum does not, however, contain all the colours that the human eyes and brain can distinguish. Brown and pink are absent, for example. See Color to understand why.

The optical spectrum includes not only visible light, but also infrared and ultraviolet.

Speed of light

The speed of light in a vacuum is exactly 299,792,458 meters per second (fixed by definition). Although some people speak of the \"velocity of light\", the word velocity is usually reserved for vector quantities, which have a direction.

The speed of light has been measured many times, by many physicists. The best early measurement in Europe is by Ole Rømer, a Danish physicist, in 1676. By observing the motions of Jupiter and one of its moons, Io, with a telescope, and 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. If he had known the diameter of the orbit in kilometres (which he did not) he would have deduced a speed of 227,000 kilometres per second.

The first successful measurement of the speed of light in Europe using an earthbound apparatus was carried out by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several thousand meters away, and placed a rotating cog wheel in the path of the beam from the source to the mirror and back again. At a certain rate of rotation, the beam could pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau measured the speed of light as 313,000 kilometres per second.