The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called light or simply light. A typical human eye will respond to wavelengths from about 390 to 700 nm, in terms of frequency, this corresponds to a band in the vicinity of 430–770 THz. The spectrum does not, however, contain all the colors that the human eyes, unsaturated colors such as pink, or purple variations such as magenta, are absent, for example, because they can be made only by a mix of multiple wavelengths. Colors containing only one wavelength are called pure colors or spectral colors. Visible wavelengths pass through the window, the region of the electromagnetic spectrum that allows wavelengths to pass largely unattenuated through the Earths atmosphere. An example of this phenomenon is that clean air scatters blue light more than red wavelengths, the optical window is also referred to as the visible window because it overlaps the human visible response spectrum. The near infrared window lies just out of the vision, as well as the Medium Wavelength IR window. In the 13th century, Roger Bacon theorized that rainbows were produced by a process to the passage of light through glass or crystal. In the 17th century, Isaac Newton discovered that prisms could disassemble and reassemble white light and he was the first to use the word spectrum in this sense in print in 1671 in describing his experiments in optics. The result is red light is bent less sharply than violet as it passes through the prism. Newton divided the spectrum into seven named colors, red, orange, yellow, green, blue, indigo, the human eye is relatively insensitive to indigos frequencies, and some people who have otherwise-good vision cannot distinguish indigo from blue and violet. For this reason, some commentators, including Isaac Asimov, have suggested that indigo should not be regarded as a color in its own right. However, the evidence indicates that what Newton meant by indigo, comparing Newtons observation of prismatic colors to a color image of the visible light spectrum shows that indigo corresponds to what is today called blue, whereas blue corresponds to cyan. In the 18th century, Goethe wrote about optical spectra in his Theory of Colours, Goethe used the word spectrum to designate a ghostly optical afterimage, as did Schopenhauer in On Vision and Colors. Goethe argued that the spectrum was a compound phenomenon. Where Newton narrowed the beam of light to isolate the phenomenon, Goethe observed that a wider aperture produces not a spectrum but rather reddish-yellow, the spectrum appears only when these edges are close enough to overlap. Young was the first to measure the wavelengths of different colors of light, the connection between the visible spectrum and color vision was explored by Thomas Young and Hermann von Helmholtz in the early 19th century
Newton's color circle, from Opticks of 1704, showing the colors he associated with musical notes. The spectral colors from red to violet are divided by the notes of the musical scale, starting at D. The circle completes a full octave, from D to D. Newton's circle places red, at one end of the spectrum, next to violet, at the other. This reflects the fact that non-spectral purple colors are observed when red and violet light are mixed.
A rendering of the visible spectrum on a gray background produces non-spectral mixtures of pure spectrum with gray, which fit into the sRGB color space.