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Any gas should give out only certain colours of light. Then how do CFLs( Compact Fluorescent Lamps)have white color. Do CFLs contain a mixture of gases?

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  • $\begingroup$ Please consider to spell out acronyms (CFL) the first time you use them. Also please give more context - where do you see the contradiction between emitting "certain colors of light" and emitting white light? E.g. red, blue and green certainly are "certain colors of light" and additively mix to yield white light. $\endgroup$ – ACuriousMind Mar 9 '18 at 16:33
  • $\begingroup$ Not all CFLs are white. Maybe the question should be, "why are some CFLs white?" $\endgroup$ – NeutronStar Mar 9 '18 at 16:35
  • $\begingroup$ White color is a mixture of colours. But any pure gas wouldn't give out all colors required to make white light so what is in CFL bulbs? $\endgroup$ – Hark Mar 9 '18 at 16:39
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    $\begingroup$ @Hark, You can make "white" by adding just three single wavelengths: en.wikipedia.org/wiki/Color_theory. P.S., The light that you see doesn't come from the gas fill in the tube. The gas emits mostly ultraviolet and some blue and some green wavelengths. The coating on the inside of the tube converts the energy to visible light by fluorescence. Hence the name, "fluorescent light." $\endgroup$ – Solomon Slow Mar 9 '18 at 17:04
  • $\begingroup$ A fun way to answer this question is to get a cheap spectrometer (or even a cheaper spectrometer) and look at the spectrum from CFLs, LEDs, modern televisions, older televisions, blue skies, white clouds, etc. $\endgroup$ – rob Mar 9 '18 at 21:33
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There are two pertinent effects here, as follows.

First, as pointed out by @ofcourseit'snotme, is that the "white" CFL tube does not contain a mixture of gases- it is coated with a mixture of phosphors, each of which radiates a different color of light when excited by the UV inside the tube. Artful blending of these phosphors can yield a variety of resultant colors- the so-called "cool white" (blue tint), "warm white" (red tint) and so on.

The second effect is metamerism, which is the mechanism built into your vision system that interprets an equal mixture of the primaries (red, green and blue) not as discrete sparkles of those colors but as white light.

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