Why can't we see individual air molecules?

Question

I always wonder, as we all know, we see the sky as blue because the air molecules scatter MOST of the light in blue range, but why can't we see individual air molecules in the atmosphere (or just anywhere like even in our surroundings)?

Most people say that it's because of the "angular resolution limit" of our eyes and so all the molecules sort of blends among each other. But the air molecules are already very far apart, so I don't think this should be the reason. And our photoreceptors can even sense a single photon. So I don't think intensity should be the limit as well.

Also, would we be able to see just one molecule in complete isolation if we have sufficient light hitting on it? Can someone please explain clearly what's actually stopping us from seeing these very tiny things even in isolation, and what is actually blending all these air molecules in the sky?

I am not an expert in this field (I just have high school knowledge), I am just curious so it would be helpful if someone explains it in simple language.

As I saw from the comments. I got this following points:

1. I was WRONG about that molecule are FAR apart. So angular resolution thing applies here.

2. Intensity also MAY play a role here.

3. Air molecules are NOT still.

But the question which then arises is let's say there is a VERY tiny molecule/object even smaller than our angular resolution limit (which Google says is almost 0.5-1 arc min.)is bright enough (i.e is able to somehow shoot enough photos for us to detect) and is STILL and is in COMPLETE isolation so there is NO other signal. So it's continuously stimulating just ONE photo receptor cell. Then what would we perceive here? Would we detect it? If yes, how would we determine its edges in order to give it a shape?

Answer 1

You are partly right - angular resolution would not be the limiting factor if the gas molecules in the air emitted light and were far enough apart to be distinguished from one another. This BBC article points out that when we see individual stars their angular size is far smaller than the resolution of our eyes. So we see stars as points of light but cannot resolve any details.

However, molecules in the air (such as oxygen, nitrogen and water) are different from stars because:

• They do not emit light, and they only reflect a very small amount of light (when you see a sunbeam from the side, you are seeing light reflected from dust particles in the air, not molecules)
• They are much closer together than you think, and they are moving more quickly than you think
• The wavelength of light in the visible spectrum ($380$ to $700$ nanometres, which is about a thousand times the size of a water molecule) places an absolute limit on the size of things we can see directly, even with the best magnification

Answer 2

It is because they are too small. If you look at a fine hair on your hand, an oxygen molecule might be one millionth of the thickness, far too small for your eye to resolve. The fact that your eye can react to faint light sources is irrelevant.

Answer 3

Edge Detection

What does it mean to "see" a molecule? I mean, every time you look into the sky, you are seeing all of the individual air molecules. I think you find that unsatisfactory because you can't distinguish between them. But you may see a tiny speck of dust floating through the air and say: "But I can see the dust particle!" What that means is that you can pick out the particle from the background of whatever scene your eyes are trained on. And in neural terms, it means that some sensors in your eye "see" the particle, and some sensors right next to them see "no particle", and your brain infers a boundary there that you interpret as "edge of particle".

So to see an individual object, you need to detect far more than a single photon. You need to detect many of them, because you need to trigger the edge detectors in your visual cortex, which receive input from numerous optical neurons.

Contrast

Secondly, there needs to be contrast between the "object" and the background. A speck of dust has high contrast because it is usually dark relative to the air it is floating through. Of course, you can't see it against a dark background, so you will only see very small dust under very bright lighting conditions. This is actually how edge detection works. The world isn't made of "lines". Our visual cortex infers the lines because some visual neurons detect light and its neighbors detect dark. If there is no contrast, there are no lines. If there are no lines, there are no edges. If there are no edges, there is no object to "see". An air molecule has no contrast relative to its neighbors. In the same way, a paint molecule on a uniformly painted wall has no contrast relative to its neighbors, which is why you can't pick them out either.

Limits

Now, suppose that we could make a single air molecule much darker or brighter than its neighbors. If it's darker, you will never know, because it will simply lower the average luminous intensity of that part of your visual field by an undetectably low amount. However, if it's bright enough, it may be detected as a point source. What will this look like? Will it "look like a molecule"? No, of course not. It will simply be a tiny fuzzy dot, bouncing around due to Brownian motion. The bouncing will make it appear much larger than it is due to persistence. But it will look similar to the faintest stars you can see, in terms of size.

The problem, however, is whether you will actually know that you are seeing this unusual anomaly. You see, vision, like every other process in the body, is noisy. Just look at a blank screen or wall. Does it look perfectly uniform? Most likely, you can notice color variations across your visual field, and tiny amounts of movement. Some people even see dots marching across their eyes when they close them or its very dark. Several factors are at play here. One is that neurons themselves are noisy, and do not fire with 100% accuracy (sometimes they will fire when they shouldn't, or fail to fire when they should). Another is persistence of vision: your visual cortex holds onto an image so it can compare movement and construct a smoothly moving visual field (which is why you can see ghosts when you switch from a bright image to a blank wall). Yet another is the fact that blood vessels run in front of your retina. Some people can see blood cells flowing across their retinas (mostly immune cells). All of these factors tend to wash out the detection of phenomena at the limits of our vision.

So, well before you reach the angular resolution limit of your eyeballs, or the cone density in your fovea, you will have to accept that many visual phenomena are simply limited by the amount of noise in the visual system, both the eyeball and the brain. The tiniest specks of dust you can see floating through the air are on the order of tens to hundreds of microns across. Air molecules are many orders of magnitude smaller than that.

Answer 4

I find the other answers sufficient, but I have a sanity check on the possibility of resolving many small objects, regardless of brightness, size, or speed, that may be persuasive.

• we resolve objects by the brightness contrast projected onto our retinas. Our cones, or rods depending on the specifics, can be thought of as pixels.
• the human retina has 5-10 million cones: $10^7$ pixels
• one cubic meter of air has roughly $10^{25}$ molecules
• with $10^{18}$ objects reflecting/emitting light per pixel, assuming perfect image focus, it is obviously impossible to resolve individual objects. The same would be true viewing macroscopic objects from far enough away that $>10^7$ are visible: for example, take a picture of a section of sand beach from 10 meters away and try to resolve individual grains.

Answer 5

Most of the following points have already been mentioned in the other answers and comments, but I think it is worth putting them together and supplying some numbers.

• It is more properly to speak here about the diffraction limit than about angular resolution. Molecules are just too small in comparison to visible light wavelengths (between 400nm and 700nm, whereas a size of molecules like $O_2,N_2,CO,CO_2$ is under a nanometer.)
• The distance between the molecules is also very small: indeed, at the sea level a cubic centimeter of air contains about $10^{19}$ molecules (see here for example), which means that intermolecular distances are less than $10^{-6}$cm$=10$nm.
• The claim that our eyes are able to detect a single photon also needs some qualifications - while a single photon generate a response in the retina, one needs a lot more photons to actually create image with sufficient contrast to make a detail perceived as visible. Many of us can see enough not to bump into walls at night at home... but it takes more light to actually see a Lego piece before stepping on it.

Answer 6

Human eye can in principle see an individual atom under suitable conditions, such as sufficient illumination and immobilization of the latter (e.g. check this image https://www.reddit.com/r/Physics/comments/7xhcu1/this_remarkable_photo_shows_a_single_atom_trapped/?rdt=63636). I see that the problem with seeing air molecules, is that:

1. They are not illuminated sufficiently (the scattered/re-emitted radiation is way too faint)

2. They move too fast. You say that "air molecules are already very far apart", but compared to what? If compared to their radii, then yes, they are well-separated. But more relevant here is their separation compared to the distance they travel during the observation time. If we take that the time resolution of our eye is about 1/24 seconds, the average distance a molecule will cover in this time at room temperature is about 3 meters, which many many orders of magnitude exceed the distance between molecules in air (not more than 100nm). So even if the molecules would glow as bright as the one in the picture in the link, they would all hopelessly blend together.

Answer 7

You bring up two very different scenarios.

Seeing air molecules in the atmosphere

Angular resolution is in fact a problem here. I don't know what basis you have for saying that air molecules are "very far apart", but the average distance between adjacent molecules in the air around you is in the ballpark of 3.5nm. That's very small.
Some estimation can eliminate the possibility of ever resolving a single molecule. In a single cubic meter of air, there are about 40 moles (or 2.4×10²⁵, or 24 millions of billions of billions) air molecules. By contrast, an eye contains about 100 million rods and cones together. Even if every single cell is capable of independently resolving a distinct object, we're clearly many orders of magnitude short of the number we need to resolve molecules in the atmosphere. There are plenty of other issues, which others have mentioned, but since you specifically brought up angular resolution I thought I'd address it.

Seeing a single molecule in a perfectly dark vacuum

Here, angular resolution is irrelevant, because there is little to no background light to resolve a small object from. This makes it possible to get pictures like this one and the one linked by John. These pictures aren't quite what you're looking for, since the particles involved are not air molecules, and the photos are somewhat magnified and likely used long exposure. Still, they do validate some of the ideas that you talked about.
At the end of the day, the requirements are:

1. The molecule is emitting enough light in the direction of your eyes to be consistently detected
2. The molecule is sufficiently still for your eyes to track it, and
3. The background is sufficiently dark

Can we meet these requirements? For small molecules such as are typical in the atmosphere, I'm not sure, but it's not totally absurd to consider. I don't think it's been done, but I bet it would be possible with enough effort, time, and money.

Answer 8

The wavelength of light places a limit on the size of an object we can see, regardless of magnification or distance. They also do not scatter or emit light very well.

Although, relative to each other and their radii, they are far apart, on a human scale, they are still hopelessly close together (one way to think about this is that $6.02\times 10^{23}$ molecules are occupying only $24\,\text{litres}$) and air molecules move incredibly fast, much faster than the "shutter speed" of our eyes.