The<unk>  (or sometimes called the optical spectrum) is the portion of the electromagnetic spectrum that is<unk>  to (can be detected by) the human eye. Electromagnetic radiation in this range of wavelengths is called<unk>  light or simply light. A typical human eye will respond to wavelengths in air from about 380 to 750 nm. The corresponding wavelengths in water and other media are reduced by a factor equal to the refractive index. In terms of frequency, this corresponds to a band in the vicinity of 400-790 terahertz. 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. Brown, pink, and magenta are absent, for example, because they need a mix of multiple wavelengths, preferably shades of red.
Wavelengths<unk>  to the eye also pass through the "optical window", the region of the electromagnetic spectrum which passes largely unattenuated through the Earth's atmosphere (although blue light is scattered more than red light, which is the reason the sky is blue). The response of the human eye is defined by subjective testing (see CIE), but the atmospheric windows are defined by physical measurement.  The<unk>  window" is so called because it overlaps the human<unk>  response spectrum; the near infrared (NIR) windows lie just out of human response window, and the Medium Wavelength IR (MWIR) and Long Wavelength or Far Infrared (LWIR or FIR) are far beyond the human response region.
The eyes of many species perceive wavelengths different from the spectrum<unk>  to the human eye. For example, many insects, such as bees, can see light in the ultraviolet, which is useful for finding nectar in flowers. For this reason, plant species whose life cycles are linked to insect pollination may owe their reproductive success to their appearance in ultraviolet light, rather than how colorful they appear to our eyes.
Visible spectrum
Absorption spectrum
Mass versus weight
Molecular orbital
Electromagnetic spectrum
Saturation 
Concentration
Spectroscopy