Why mercury vapor is colorless whereas potassium's one not?

submitted by ZephirAWT from youtube.com

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[–]ZephirAWT[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (1 child)

This random clip of otherwise amazing YouTube channel of @ChemicalForce brought my attention (here in more details). This is because potassium vapor on this video exhibits eerily bluish-green color, which is not accidentally complementary to rose-pink color of potassium dyed flame (apart of yellow streaks of sodium impurities from surrounding dust particles entering the flame). With compare of it mercury vapor absorbs emission spectrum of mercury at least as much as potassium, its complementary color should be therefore visible on sunlight for naked eye. But as we know, mercury vapor remains colorless even in thick layer.

One explanation can be in that what we can see on the wall in this video isn't actual color of mercury lamp itself, but the color of the fluorescence of the wall surface. Which would imply that cold mercury vapor blocks invisible UV radiation only - not the visible one. Second, the absorbtion lines within visible part of mercury lamp spectrum are excited only by high temperatures of mercury lamp discharge, whereas at low temperatures they remain inactive due to higher ionization (output) work of these mercury lines in comparison to potassium.

[–]ZephirAWT[S] 1 insightful - 1 fun1 insightful - 0 fun2 insightful - 1 fun -  (0 children)

Methylene blue also isn't always just blue - its powder reflects light in complementary color because blue portion of light gets absorbed during specular reflection from it. Analogously methyl-violet dye isn't actually violet in crystalline form - but golden green in color.

The Blood in Your Veins Isn't Actually Blue This is like to say, that Earth atmosphere isn't actually blue - but yellow or red. This is actually what we would see during Earth's eclipse of Moon.

This is what the earth looked like from the blood 'supermoon'

The memo is, every matter has actually two complementary colors assigned: this one corresponding scattered and absorbed light and it's just a matter of common practice, which perspective (intrinsic or extrinsic one) is more dominant and as such considered natural for color impression.