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Monochromatic radiation can be produced by a number of methods. Isaac Newton observed that a beam of light from the sun could be spread out by refraction into a fan of light with varying colors; and that if a beam of any particular color was isolated from that fan, it behaved as "pure" light that could not be decomposed further.
While no source of electromagnetic radiation is purely monochromatic, in practice, it is usually used to describe very narrowband sources such as monochromated or laser light. The degree of monochromaticity can be defined by the spectral linewidth). A device which isolates a narrow band of light from a broadband source is called a monochromator.
A source, like the sun, which is very far away, provides collimated light. Newton used sunlight in his famous experiments. In a practical monochromator, however, the light source is close by, and an optical system in the monochromator converts the diverging light of the source to collimated light.
Many sources for monochromatic light can be used. Most lasers emit light of a very narrow bandwidth, and often provide a suitable light source. A helium–neon laser emits light at 632 nanometres (red), while a frequency doubled Nd:YAG laser emits light at 532 nm (green).
To produce coherent light he passed the monochromatic light from an emission line of a mercury-vapor lamp through a pinhole spatial filter. In February 2011 it was reported that helium atoms, cooled to near absolute zero / Bose–Einstein condensate state, can be made to flow and behave as a coherent beam as occurs in a laser.
The basic characteristics distinguishing it from the Michelson configuration are the use of a monochromatic point light source and a collimator. Michelson (1918) criticized the Twyman–Green configuration as being unsuitable for the testing of large optical components, since the available light sources had limited coherence length.
Light usually has multiple frequencies that sum to form the resultant wave. Different frequencies undergo different angles of refraction, a phenomenon known as dispersion. A monochromatic wave (a wave of a single frequency) consists of successive troughs and crests, and the distance between two adjacent crests or troughs is called the ...
By definition, if one constructs a light source that emits monochromatic green light with a frequency of 540 THz, and that has a radiant intensity of 1/683 watts per steradian in a given direction, that light source will emit one candela in the specified direction.