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I think that something is invisible if it's isolated particles are smaller than the wavelength of visible light. Is this correct?

Why is air invisible? What about other gases and fumes which are visible?

Manishearth
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Kit
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7 Answers7

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I think the pithy answer is that our eyes adapted to see the subset of the electromagnetic spectrum where air has no absorption peaks. If we saw in different frequency ranges, then air would scatter the light we saw, and our eyes would be less useful.

Zo the Relativist
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    There is plot in Jackson showing the absorptivity of light in water as a function of frequency. The optical band fits neatly into a deep trough. – dmckee --- ex-moderator kitten Dec 12 '10 at 02:34
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    Well, it's good that water vapor is a trace part of the atmosphere, then? Also, are those absorption peaks still valid when you go from liquid water to vapor, anyway? Water clearly has very different optics from water vapor. – Zo the Relativist Dec 12 '10 at 02:55
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    @Jerry: Note the sense here: low absorptivity means good transmission: i.e. I agree with you. I think that vapor and liquid have rough the same properties discounting droplets, which obviously have a macroscopic refractive behavior. – dmckee --- ex-moderator kitten Dec 12 '10 at 03:16
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    Heh, I inverted absorptivity and emmissivity. – Zo the Relativist Dec 12 '10 at 03:45
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    @Jerry: Do you mean that air reflects/refracts/absorbs/scatters electromagnetic radiation at the invisible ranges more than it does with visible light? – Kit Dec 13 '10 at 01:44
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    Yes. And dmckee's comment corroborates that. – Zo the Relativist Dec 13 '10 at 04:09
  • This is a good example of the anthropic principle. – TRiG Dec 14 '10 at 20:55
  • @dmckee: Which paper by Jackson do you mean? – Kit Mar 17 '11 at 07:02
  • @Kit: The popular E&M textbook. – dmckee --- ex-moderator kitten Mar 17 '11 at 11:22
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    @TRiG More like a good example of evolution. Creatures with sensory organs that don't detect as well are less likely to survive (statistically) than creatures with better sensory organs, so over time the creatures that can actually see through air and water will be the vast majority. (In those areas where such vision is useful/necessary for survival, of course.) – JAB Nov 29 '16 at 16:32
  • @JAB True, but that just explains why we have sight at all. You still need a mechanism for sight, and it must be sensitive to light that carries enough information to be useful (-> high power, high frequency), but also isn't too destructive (-> low frequency). Visible light is in the band where plenty of single bonds fit well and you don't get much thermal noise, so you get a plethora of colours with decent resolution (extra information!), while the energy is too low to break the stronger bonds our life depends on (so your DNA doesn't break as it does with UV light). – Luaan Nov 30 '16 at 12:46
  • Michiou Kaku says in his book that in his childhood he wondered that fish like us could not see the water. – Kashmiri Jul 04 '20 at 16:47
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I don't think one can just state that particle size smaller than the wavelength of light implies no interaction with light. You have to look at the quantum mechanical modes of the atoms/molecules. If they have modes with frequencies in the wvaelength range you are interested in, then you will get interaction/absorption. I think that even clean air does have nonzero absorption in the visible. Have a large enough column density along a beam of light, and there will be some absorption.

Also do note, that the speed of light is slower in air than in a vacuum, so the air does have some effect. You do get effects of refraction, such as mirages, and heat waves seen looking across convection over a hor surface. Also if you point a telescope at a star, you see the mess the atmosphere makes of the image.

Omega Centauri
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Even without any absorption peaks, there is Rayleigh scattering

http://en.wikipedia.org/wiki/Rayleigh_scattering

which works on particles much smaller than wavelength of light, such as gas molecules. Rayleigh scattering is basically why sky is blue (and why it would be blue even if the air contained no oxygen). The scattering is inversely proportional to wavelength to 4th power, so the blue light is scattered a lot more than red light.

Dmytry
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Air is very sparse compared with liquids and solids, so there's just much less material to block light. Secondly, air is very uniform and homogenous so there are no edges to cause reflections and scattering, except in the extreme case of a explosion shockwave. For the same reason, still water is mostly invisible except at the surface, while frothy water is very visible. Thirdly, the molecules that comprise air are very simple and don't have a lot of lines in the visible range. This is in contrast to complex organic molecules with a lot of vibrational and rotational modes. Also, our eyes evolved to see in a range where air is transparent.

Hermione
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Air is mostly composed of first two row small molecules like O2 and N2, and atoms like Argon. For all these, the absorption is in the deep ultraviolet. Molecules which absorb in the visible have smaller energy differences in their first absorption bands, although chlorine(2) and Iodine(2) molecules are visible. This answer is meant to be complementary to and supportive of the evolutionary answer given above

sigoldberg1
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Simplistic answer: But it isn't invisible; it can bee seen from outside - just look at photos from the ISS or shuttle. It is just that the observer standing inside it doesn't see it unless something unusual occurs such as fire or fumes.

Lindsay
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The air has purely real rifraction indix $\sim 1$, no dispersive, "like vacuum". So there is not absorption, and the air is transparent for visible wavelenght, like X rays for metals over $plasm frequency$.

Boy S
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    There is a sense in which you've just restated "air is transparent" in formal language. Not that you're wrong or anything, but I suspect the OP is looking for some micro-physics...I was thinking about discussing particle size; particle spacing; the lack of free charges; and atomic and molecular absorption spectra. But I'm not sure I can be right enough for my own satisfaction. – dmckee --- ex-moderator kitten Dec 12 '10 at 02:24