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This question already has an answer here:

Why are gases difficult to see?

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marked as duplicate by Kyle Kanos, JamalS, Brandon Enright, John Rennie, Martin Jan 9 '15 at 9:18

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    $\begingroup$ the simple, obvious, and uninformative answer is because light goes through them $\endgroup$ – Jim Jan 8 '15 at 21:31
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    $\begingroup$ Not all gases are invisible. Chlorine gas is a pale yellow/green colour. $\endgroup$ – theo Jan 8 '15 at 22:24
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    $\begingroup$ In a different sense, normal atmospheric gases are hard to see because the evolution of eyes that could not see through the gases would not be very useful. If you could see in the infrared, you would not have problems detecting air. $\endgroup$ – Jon Custer Jan 8 '15 at 22:52
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Chemical factors

The more "localized" the electrons are the higher frequencies they naturally vibrate at (like a shorter guitar string playing a higher note). Gases must be simple, small molecules otherwise they would condense. Small molecules can't have electrons that are delocalized over many atoms. All substances have tightly localized electrons that let them absorb deep ultraviolet and low-energy xray light. Most dyes, however, are complex molecules with delocalized electrons that allow longer-wavelength visible light absorption.

There are other ways that objects, including certain gases, can absorb visible light, but they are the exceptions rather than the rule.

Physical factors

Gases are 1000 times less dense.

Gases alone can't form heterogeneous structures. Water is clear but snow and foam are white because the refractive mismatch between ice/water and air scatters light in random directions. Any two gases when mixed (unless they react) will form a homogenous mixture.

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    $\begingroup$ Isn't there also a biological factor? Like since we are surrounded by gases, we developed sensors ("eyes") that are able to see through them? $\endgroup$ – Twinkles Jan 9 '15 at 8:10
  • $\begingroup$ Yes, but: there isn't that wide a range for which photochemical sensors can work, and the atmosphere is transparent enough over most of this range. Vision at other wavelengths, like snake pits, has very low resolution. $\endgroup$ – Kevin Kostlan Jan 9 '15 at 14:47
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Most obviously, gases contain not many atoms per cubic meter. There is a lot of space between them, and the photons will just pass unimpeded.

A second effect is that light, being an electro-magnetic wave is affected by electric fields and conductivity. Metals reflect light, which makes them obviously not transparent. This is directly because metals conduct electricity. Even for non-conductive solids, they're generally solid because all the atoms stick closely together by electro-magnetic forces (not gravity nor nuclear forces). In gases, the atoms don't stick together because the electro-magnetic forces are weaker, and so these forces also have less influence on photons.

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Gases are transparent to light, just as glass is transparent. Some gases are coloured, such as chlorine which is greenish, and bromine vapour, which is brownish.

Which begs the question, why are some substances transparent to light. And that is a good question.

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