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I am not sure what causes gas molecules to be invisible.This question may look silly but I really want to know the story behind it.

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Who said that we can't see gas molecules? –  Sachin Shekhar May 5 at 11:15
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@Awesome That is not really an explanation, right? –  Bernhard May 5 at 11:22
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The total mass of the gas column you're looking through matters. For example, you have probably noticed that air is blue. –  rob May 5 at 14:18
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@iamnotmaynard That site is basically valid, if somewhat confusing. The blue sky is due to blue being scattered, rather than the other colors being absorbed (as is the case for , say, a blue sheet of paper). If there were no scattering, the sun would be brighter and the rest of the sky would appear black. However, the non-scattered light (or sunset colors) is not a result of the color of the atmosphere but just the remains of sunlight after some colors have been scattered away from the direct path. –  Carl Witthoft May 5 at 16:01
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Ask yourself, 'invisible to whom?' Visibility is subjective to an observer. –  Henk Langeveld May 6 at 7:29

9 Answers 9

up vote 35 down vote accepted

First of all, gas molecules are not invisible. There are plenty of elements whose gaseous state is quite colored, but these (iodine, e.g.) are in such rare amounts in the atmosphere that the net effect is not discernable to the eye. Next, if you Google for "atmospheric transmission curves," you'll see all sorts of spectral absorption going on, again at rates which aren't normally detectable by your eye.

As it happens, the more prevalent species (nitrogen, oxygen, CO2, etc) do not absorb or reflect significantly over the visible spectrum. That's partly (although not entirely -- this becomes a biological rather than physical question) why our eyes see in the range they do.

EDIT: per @DavidRicherby's request adding: these gases do not absorb because they have no resonances or electron shell gaps to match -- or as everyone's said, because what absorption cross-section they have is small enough that the net effect is not distinguishable to our eyes

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Even if they do reflect light, can we say that they are not detectable to our eyes (because gas molecules are so small (about $2X10^{-9} m$ or even smaller) and apart)? –  Godparticle May 5 at 12:03
    
@Godparticle I suppose so, but invisibility by size applies to all molecules. I was interpreting the question as "why doesn't a big blob of (pick a gas) have a color we can see?" –  Carl Witthoft May 5 at 12:44
    
(+1) Thank you for the explanation. I understood what you meant:) –  Godparticle May 5 at 13:40
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+1 for a biological answer - the creatures whose vision was not blocked by gasses were the ones who lived to breed, I would imagine. –  corsiKa May 5 at 21:14
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Anthropic Principle! –  Aron May 8 at 7:16

Nitrogen Dioxide (photograph credit: Efram Goldberg)
[Note: left-most ampule is cooled to -196°C and covered by a white layer of frost.]

$NO_2$ is a good example of a colorful gas. $N_2O_4$ (colorless) exists in equillibrium with $NO_2$. At lower temperature (left in Wikipedia photo), $N_2O_4$ is favored, while at higher temperature $NO_2$ is favored.

For a gas to have color, there needs to be an electronic transition corresponding to the energy of visible light.

$F_2$ (pale yellow), $Cl_2$ (pale green), $Br_2$ (reddish), and $I_2$ (purple) are other examples of gases with color.

A complete analysis of how visible or invisible a gas is would consider the density of the gas, the length of the light path, the Rayleigh scattering function of the gas, and the absorbance coefficients of any electronic transitions availible to the gas molecules or atoms in the visible range.

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The $N_2O_2$ appears to be more white than colourless in that image. –  naught101 May 8 at 23:45
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@naught101 there is some frost on the outside of the ampule at the extreme left because it is -196 degrees C. The rest are 0 C to 50 C. –  DavePhD May 9 at 0:09
    
Ah, ok. However, even the one second from the left looks a little more opaque than the other three (it's harder to see the back of the ring). –  naught101 May 9 at 0:11
    
could be some condensation on the second one at 0 degrees C –  DavePhD May 9 at 0:14

As has been said by many answers; all gases aren't colourless, for example chlorine gas is a pale yellow; which is a good things as its very dangerous.

So the gases in our atmosphere are colourless. But this is completely the wrong way round to look at it. If our eyes operated at frequencies that were blocked by gases in the atmosphere they wouldn't work very well. And this is an important point because the gases in our atmosphere aren't transparent at all frequencies. For example this is the absorption spectrum of water vapour:

enter image description here

reproduced from http://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water#Atmospheric_effects

If our eyes operated around 100nm we would live in a very dark world, almost all the light would be absorbed by the atmosphere. The same if they operated at 10 micrometres. But our eyes evolved to use the light that was available to them; and that light was between 400-700nm; right in the middle of that drop in the absorption (obviously you'd need to look at nitrogen’s and oxygen’s absorption spectra as well to get a full picture).

So the reason we can't see common gases; because evolution optimised out eyes to work that way. Had we evolved in an atmosphere mostly made of chlorine gas I would wager that we would still be asking "Why can't we see gases?" and someone would come up with the counter examples of how the (on their world) rare gases water vapour, oxygen and nitrogen were visible.

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You forgot to add the disclaimer in the past 6000 years :-) j/k. –  ja72 May 6 at 14:57
    
This is likely just another reason the earth is so perfect: optical window in the atmosphere matches the black body radiation curve of the sun! –  Phil May 7 at 18:34
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@Phil observer bias. A planet that couldn't have life on it wouldn't have life on it to go "How remarkable that this planet supprts life" –  Richard Tingle May 7 at 18:36

Some gases actually are visible (nitrogen dioxide for instance). The air is invisible, because its molecules don't absorb the visible light. These molecules simply don't have useful vibration modes available to absorb these wavelengths, or the electrons in their orbitals can't utilize the frequencies of visible light to move to higher orbital (the energy differences do not correspond to visible light).

In some other part of the electromagnetic spectrum the air could be visible.

One of the reasons why the eyes became sensitive in the "visible" spectrum is that air does not absorb there. Otherwise the eyes would be useless: you would not see anything but air. Our eyes can tell us what is happening around only if they use the part of spectrum where air does not absorb.

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This is, as I mentioned, only part of the story. It turns out there are only certain wavelength ranges to which the classes of chemicals that animals can produce are sensitive. There are other spectral bands with high atmospheric transmissivity but no organic compound to detect them. –  Carl Witthoft May 5 at 12:46
    
@CarlWitthoft You are correct. I did not elaborate on that in detail, but I'll probably update my answer. –  mpv May 5 at 15:30
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@mpv dinitrogen tetraoxide is not visible, nitrogen dioxide is. –  DavePhD May 6 at 15:37

One factor to keep in mind is that for a low-density material with relatively weak interactions with light, the total mass of the column that light passes through will make a big difference in the perceived color. For instance, if you fill a white bathtub with water, you'll notice that a centimeter-scale column of water from the tap (or from your water glass) is transparent, while the decimeter-scale column in the bottom of the tub is distinctly blue.

thicker water columns have more obvious color

You can see the same effect if you look at a green or brown mountain from a few tens of miles away: the greens and browns are washed out by the blue color of the many tons of intervening air.

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Why are liquids invisible? And why are gases like silvery blobs? (...asks a creature who spent their entire life underwater.)

Gases are transparent, not invisible. Life at the bottom of an 'ocean of air' can give certain air-breathing organisms a distorted viewpoint.

If we spent our lives in vacuum, then we'd think that both air and water were transparent fluids. We'd notice that air does bend light far less than the water does. In a vacuum environment, a clear bag of air would behave less like a lens, if compared to a clear bag of water.

Actual classroom demonstration: get an aquarium full of water. Fill a water-balloon. Now hold the balloon submerged in the aquarium, and let it release the water. See anything? Nope. This obviously proves that water is invisible. :) And if we had a gas-filled environment, and then released the contents of a gas-filled balloon, we could prove to ourselves that gas is invisible. No? We are airfish, living at the bottom of the nitrogen ocean, and firmly convinced that gas is an invisible material.

Here's yet another perspective: suppose that you're about 1000KM tall. You bend down, cup your hands, and scoop up some of Earth's atmosphere. Lift it high into the vacuum. It looks like translucent light-blue smoke! The KMs-deep pool of air in your hands makes your palms a bit hard to see. Pour it out again, and as it falls it forms a brilliant sky-blue plume against the blackness of space. Obviously air is far from invisible.

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Fill a balloon with air, hold it below the water surface of the aquarium, and release the air. Is air invisible? :) –  Michael Kjörling May 6 at 7:56
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Fill a balloon with water, hold it above your head, and release the water. Is water invisible? –  dotancohen May 6 at 15:39

Gas can be hugely visible. The sun is all made of gas and is totally intransparent. Inside the sun light particles (photon) travel only centimeters (in the very deep) to kilometers (nearer the surface) before being absorbed. Not really different than other "particles" of the local gas. So you can't see into the sun in light (you can using acoustic waves as subsurface diagnostc but that is another story).

What we call "the solar surface" is the layer far out where the gas gets tenuous enough to become transparent. There the photons escape as sunlight. The gas there is actually much less dense than the transparent air around us because it is made up of nearly pure hydrogen (making it quite opaque to visible light if sufficient hydrogen atoms grab an extra (second) electron, a process only understood in the 1940s).

A small fraction of the very small fraction that happens to strike the earth gets scattered in our atmosphere; those that happen to bounce towards your eye make up the blue sky that you see. Blue not because they change in energy (color), only because more photons get scattered in the blue than in the red - so the sun shows red at sunset because more blue went out of the direct way to your eye.

The question is good because intransparency of gases appears counter-intuitive to us. This is why "radiative transfer in stellar atmospheres" is an advanced topic in astrophysics courses. The light that comes out of stars is our main diagnostic to understand them, but interpreting this light needs good appreciation of the stellar gas' intransparency. Google this topic and read my lecture notes...

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The Sun is producing its own light which would overpower any semblance of light coming from the other side even if it is totally transparent. –  Lie Ryan May 5 at 22:23
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Its worth noting that (the vast majority) of the sun is not a gas. It is a plasma; the forth state of matter where the electrons are stripped entirely from the nuclei –  Richard Tingle May 6 at 13:55
    
@Richard Tingle - Yes, indeed only at the bottom of the solar atmosphere, precisely the layer where visible light escapes, is the gas (mostly hydrogen molecules) neutral, with one-tenth of a promille having a second electron and governing the escape of the solar radiation we see. At larger depth into the sun the gas is increasingly ionised; in the core it is indeed completely ionised (all electrons off). Still a "gas" since it still obeys the simple "ideal gas law" P = NkT. –  Rob Rutten May 26 at 20:14
    
Saying a plasma is a kind of gas is like saying a gas is a kind of liquid because it doesn't have a defined shape. They are very different beasts; obviously that behave very differently under electric and magnetic fields but more subtly they have collective interations over a long range and can move "as a bulk" whereas gas interactions are always 2 particle interactions. See this wiki page, specifically the section explaining the difference between a gas and a plasma: en.m.wikipedia.org/wiki/Plasma_(physics) –  Richard Tingle May 26 at 20:24

I just had to interject here!

In the expansion of your question you ask that

I am not sure what causes gas molecules to be invisible

Well, all "molecules" are invisible to our eyes, we just don't have the resolving power to see them, if you have an atomic force microscope you can see them like this

However you can see many gases in general like @DavePHD has clearly shown!

If you still intend to talk about the fact that you can see pretty much all solids or liquids and not all gases, then you must take a look at people banging themselves in mirrors or glasses as those too become invisible to us at various occasions.

While pretty much all solids and liquid are organised enough to at least reflect light, gases are too scattered to do that! The only property which allows gases to become visible is the absorption or emission of photons, if during absorption the complementary light is in visible range we can see the gas, and if emitted light is in visible range we can see it, otherwise we just can't not with our eyes!

In the last paragraph, do not think about fog or other such things that look like gases and say that those reflect! There are other phenomenons which play there and furthermore fog is not gas! Reflection only takes place from gases when it is impure and is more of a colloidal nature, as it is in smoke that the pollutant particles make it look black/grey/white!

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Visibility is subjective

Visibility is subjective, you need an observer.

You asked for the story. It starts with our earliest ancestors, who developed sensors that were sensitive to electromagnetic radiation.

What kind of sensors and what kind of radiation? Whatever made a difference.

In the beginning? Whatever radiation was available, whatever got through the atmosphere with sufficient energy to reach the surface of the earth.

As the atmosphere changed, so did the sensors adapt to the radiation that would get through.

Over time, those sensors evolved into eyes. As they did with many other species.

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