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intensity (or brilliance or amplitude, perceived by humans as the brightness of the light),
frequency (or wavelength, perceived by humans as the color of the light), and
polarization (or angle of vibration and not perceptible by humans under ordinary circumstances)
Due to wave-particle duality, 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
Visible light is the portion of the electromagnetic spectrum between the frequencies of 380 THz (3.8×1014 hertz) and 750 THz (7.5×1014 hertz). The speed (c), frequency (f or ?), and wavelength (?) of a wave obey the relation:

Because the speed of light in a vacuum is fixed, visible light can also be characterised by its wavelength of between 400 nanometres (abbreviated 'nm') and 800 nm (in a vacuum).

Light excites the rod cells and cone cells in the retina of the human eye, creating electrical nerve impulses that travel up the optic nerve to the brain, producing vision.

Speed of light
Main article: Speed of light
Although some people speak of the "velocity of light", the word velocity should be reserved for vector quantities, that is, those with both magnitude and direction. The speed of light is a scalar quantity, having only magnitude and no direction, and therefore speed is the correct term.

The speed of light has been measured many times, by many physicists. The best early measurement is Ole Rømer's (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 a speed of 227,000 kilometres per second (approximately 141,050 miles per second).

The first successful measurement of the speed of light using an earthbound apparatus was carried out by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several thousand metres 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.

Léon Foucault used rotating mirrors to obtain a value of 298,000 km/s (about 185,000 miles/s) in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's results in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 186,285 mile/s (299,796 km/s [1,079,265,600 km/h]). In daily use, the figures are rounded off to 300,000 km/s and 186,000 miles/s.

Refraction
Main article: Refraction
All light propagates at a finite speed. Even moving observers always measure the same value of c, the speed of light in vacuum, as c = 299,792,458 metres per second (186,282.397 miles per second). When light passes through a transparent substance, such as air, water or glass, its speed is reduced, and it undergoes refraction. The reduction of the speed of light in a denser material can be indicated by the refractive index, n, which is defined as:

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