Optical spectrum : Visible 

The optical spectrum (visible spectrum) is the portion of the electromagnetic spectrum that is visible to the human eye (see color for the sequence of colors). There is no exact bound to the visible region of the electromagnetic 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 term "optical spectrum" originally applied only to that region of the electromagnetic spectrum which is visible to the normal human eye, but is now considered to include all wavelengths between the shortest wavelengths of X-rays and the longest of radio. At this writing, no formal spectral limits are recognized nationally or internationally.

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.

See frequency, wavelength, Rydberg formula.

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