Spectral colors

The familiar colors of the rainbow in the spectrum Ц named using the Latin word for appearance or apparition by Isaac Newton in 1671 Ц include all those colors that can be produced by visible light of a single wavelength only, the pure spectral or monochromatic colors. The table at right shows approximate frequencies (in terahertz) and wavelengths (in nanometers) for various pure spectral colors. The wavelengths are measured in air or vacuum (see refraction). The color table should not be interpreted as a definitive list Ц the pure spectral colors form a continuous spectrum, and how it is divided into distinct colors linguistically is a matter of culture and historical contingency (although people everywhere have been shown to perceive colors in the same way[2]). A common list identifies six main bands: red, orange, yellow, green, blue, and violet. Newton's conception included a seventh color, indigo, between blue and violet. Optical scientists Hardy and Perrin list indigo as between 446 and 464 nm wavelength.[3] The intensity of a spectral color, relative to the context in which it is viewed, may alter its perception considerably; for example, a low-intensity orange-yellow is brown, and a low-intensity yellow-green is olive-green. For discussion of non-spectral colors, see below. A rainbow is an optical and meteorological phenomenon that is caused by reflection of light in water droplets in the Earth's atmosphere, resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the sun. In a "primary rainbow", the arc shows red on the outer part and violet on the inner side. This rainbow is caused by light being refracted while entering a droplet of water, then reflected inside on the back of the droplet and refracted again when leaving it. In a double rainbow, a second arc is seen outside the primary arc, and has the order of its colours reversed, red facing toward the other one, in both rainbows. This s cond rainbow is caused by light reflecting twice inside water droplets. The visible spectrum is the portion of the electromagnetic spectrum that is visible to (can be detected by) the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm.[1] In terms of frequency, this corresponds to a band in the vicinity of 400Ц790 THz. A light-adapted eye generally has its maximum sensitivity at around 555 nm (540 THz), in the green region of the optical spectrum (see: luminosity function). 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. Visible wavelengths pass through the "optical window", the region of the electromagnetic spectrum which 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 called 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 though other animals may experience them. Many species can see light with frequencies outside the "visible spectrum," which is defined in terms of human vision. Bees and many other insects can detect ultraviolet light, which helps them find nectar in flowers. Plant species that depend on insect pollination may owe reproductive success to their appearance in ultraviolet light, rather than how colorful they appear to humans. Birds, too, can see into the ultraviolet (300Ц400 nm), and some have sex-dependent markings on their plumage that are visible only in the ultraviolet range.