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Essay on Color, Para. 2, reality in physics (continued)
2.4 Some Spectra
As opposed to "spectral chracteristics", I call a "spectrum" not the light power distribution function, but the distributed light itself.
This kind of spectrum can be seen magnificently in nature; it's called a rainbow. Here is just an artist's view of it, showing somewhat exaggerated saturations:
Not as magnificent, but with even more astoundingly vivid colors, you can see spectra in the laboratory. A "polychromator" will produce a ribbon of consecutive monochromatic colors, to which you can even assign a wavelength scale:
(Sorry: This publication uses RGB-colors and cannot reproduce full color vividness.)
Linking less "continuous" light sources (e.g. low pressure gas discharge lamps) to your polychromator, you'll get a spectrum ribbon containing dark gaps between some colors. And taking a laser, your "ribbon" will contain just one single (emission-)line of colored light, while the rest of the ribbon is lost in the dark.
Analysing sunlight, Fraunhofer found narrow dark gaps in the spectrum which are caused by certain atom and molekule species absorbing the light selectively. These (absorption-)lines are called Fraunhofer lines.
How does a polychromator work, and what is a monochromator? - Just look at "prism" and "grating"!