<P> The spectrum does not, however, contain all the colors that the human eyes and brain can distinguish . 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 also called pure colors or spectral colors . </P> <P> Visible wavelengths pass through the "optical window", the region of the electromagnetic spectrum that allows wavelengths to pass largely unattenuated through the Earth's atmosphere . An example of this phenomenon is that clean air scatters blue light more than red wavelengths, and so the midday sky appears blue . The optical window is also referred to as the "visible window" because it overlaps the human visible response spectrum . The near infrared (NIR) window lies just out of the human vision, as well as the Medium Wavelength IR (MWIR) window, and the Long Wavelength or Far Infrared (LWIR or FIR) window, although other animals may experience them . </P> <P> In the 13th century, Roger Bacon theorized that rainbows were produced by a similar process to the passage of light through glass or crystal . </P> <P> In the 17th century, Isaac Newton discovered that prisms could disassemble and reassemble white light, and described the phenomenon in his book Opticks . He was the first to use the word spectrum (Latin for "appearance" or "apparition") in this sense in print in 1671 in describing his experiments in optics . Newton observed that, when a narrow beam of sunlight strikes the face of a glass prism at an angle, some is reflected and some of the beam passes into and through the glass, emerging as different - colored bands . Newton hypothesized light to be made up of "corpuscles" (particles) of different colors, with the different colors of light moving at different speeds in transparent matter, red light moving more quickly than violet in glass . The result is that red light is bent (refracted) less sharply than violet as it passes through the prism, creating a spectrum of colors . </P>

Which group of colors cannot be found in the spectrum