Download 2.2 The visible spectrum

2.2 The visible spectrum
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 400–790 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 only be made by a mix of multiple
wavelengths.
Visible wavelengths also pass through the "optical window",
the region of the electromagnetic spectrum that passes largely
unattenuated through the Earth's atmosphere. Clean air scatters blue light
more than wavelengths toward the red, which is why the mid-day sky
appears blue. The human eye's response is defined by subjective testing,
but atmospheric windows are defined by physical measurement.
The "visible window" is so called because it overlaps the human visible
response spectrum.
The near infrared (NIR) windows lie just out of the 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.
Many species can see frequencies which fall outside the "visible
spectrum". Bees and many other insects can see light in the ultraviolet,
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 (300–400 nm), and some have sexdependent markings on their plumage, which are only visible in the
ultraviolet range.