The visible portion of the electromagnetic spectrum, perceived either as white light (light which contains each wavelength of color in equal amounts and thus lacks any particular hue) or as light of a particular color, depending on which wavelengths are dominant. The portions of the electromagnetic spectrum which surround the visible spectrum—namely, the infrared and ultraviolet—are also occasionally referred to as "light," although they cannot be detected visibly. Ultraviolet light is occasionally—and not entirely correctly—referred to as black light.

Depending upon the experiment, light can either be a wave, as was first proposed by Dutch physicist Christian Huygens, or as a series of discrete particles, as was first proposed by Sir Isaac Newton. The particulate nature of light was the predominant theory of the nature of light until the early 1800s, when Thomas Young and Augustin-Jean Fresnel demonstrated in experiments the diffraction and interference of light, which could only occur if light was a wave. In 1864, James Clerk Maxwell demonstrated that light was a form of electromagnetic radiation, which boosted support for the wave theory of light. However, in 1905, Albert Einstein reconceived the notion of light as discrete particles (or photons, or quanta) in order to explain the photoelectric effect, or the emission of electrons produced when light strikes an object. Eventually, it was decided that light could be both a wave and a series of particles.

The intensity (or luminosity) of a light source is measured in candles. The intensity of light reflected from a surface (or luminance) is measured in candles per square meter, or foot candles, or footlamberts. (See Color.)

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