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de:Lichtspektrum
Prism showing spectrum of visible light.
The optical spectrum (visible light or visible spectrum) is the portion of the electromagnetic spectrum that is visible to the human eye. The optical spectrum is a composite, or mixture, of the various colors.
There are no exact bounds to the optical spectrum ; a light-adapted eye typically has a maximum sensitivity of ~555 nm (in the green). Commonly the response of the eye is considered to cover 380 nm to 780 nm although a range of 400 nm to 700 nm range is more common. The eye may, however, have some visual response at even wider wavelength ranges.
Wavelengths in the range visible to the eye occupy most of the "optical window", a range of wavelengths that are easily transmitted through the Earth's atmosphere.
Note: Ultraviolet and infrared are often considered to be "light" but are generally not visible to the human eye.
The optical spectrum and Isaac Newton
It was Sir Isaac Newton in 1666 who first used the word spectrum to refer to the celebrated Phenomenon of Colours which can be extracted from sunlight, by a glass prism.
Visible light
Visible light is electromagnetic radiation that is made of colors of light that the eye can see. This light has wavelengths that are generally expressed in nanometers and/or angstroms.
A prism can separate an incoming white light into its constitutive colors : this is because the glass of which the prism is made is a dispersive medium (ie the optical index n depends on the wavelength). Thus, blue color is more sharply bent than the red color in a prism.
See also
spectrum, frequency, wavelength, Rydberg formula.
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