In the 17th century, Isaac Newton discovered that prisms could disassemble and reassemble white light, and described the phenomenon in his book Opticks. In the 13th century, Roger Bacon theorized that rainbows were produced by a similar process to the passage of light through glass or crystal. This reflects the fact that non-spectral purple colors are observed when red and violet light are mixed. Newton's circle places red, at one end of the spectrum, next to violet, at the other. The circle completes a full octave, from D to D. The spectral colors from red to violet are divided by the notes of the musical scale, starting at D. History Newton's color circle, from Opticks of 1704, showing the colors he associated with musical notes. The near infrared (NIR) window lies just out of the human vision, as well as the medium wavelength infrared (MWIR) window, and the long-wavelength or far-infrared (LWIR or FIR) window, although other animals may perceive them. The optical window is also referred to as the "visible window" because it overlaps the human visible response spectrum. An example of this phenomenon is when clean air scatters blue light more than red light, and so the midday sky appears blue (apart from the area around the Sun which appears white because the light is not scattered as much). Visible wavelengths pass largely unattenuated through the Earth's atmosphere via the " optical window" region of the electromagnetic spectrum. Colors containing only one wavelength are also called pure colors or spectral colors. Unsaturated colors such as pink, or purple variations like magenta, for example, are absent because they can only be made from a mix of multiple wavelengths. The spectrum does not contain all the colors that the human visual system can distinguish. The optical spectrum is sometimes considered to be the same as the visible spectrum, but some authors define the term more broadly, to include the ultraviolet and infrared parts of the electromagnetic spectrum as well. Under optimal conditions these limits of human perception can extend to 310 nm ( ultraviolet) and 1100 nm ( near infrared). These boundaries are not sharply defined and may vary per individual. In terms of frequency, this corresponds to a band in the vicinity of 400–790 terahertz. A typical human eye will respond to wavelengths from about 380 to about 750 nanometers. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. White light is dispersed by a prism into the colors of the visible spectrum. For The Dear Hunter album, see The Color Spectrum.
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