Some quick napkin math (please let me know if I'm way off base here): in the second pic, the electrodes are ~74 pixels apart and the strontium atom is ~2 pixels wide.
The electrodes are ~0.078 inches apart, so these 2 pixels represent ~0.0021 inches (or ~0.05334 mm).
Google tells me that the Van der Waals radius of a single strontium atom is 255 pm.
So the diameter of the actual atom is something like 10,000x smaller than the space represented by these pixels. Crazy!
The atom was illuminated and irradiated light for a long enough time to register on the sensor.
That is not true. It might be noisy, but it would show up.
In point of comparison the human eye can see a single photon.
A single atom can emit a single photon, ergo you can see a single atom.
You won't get any detail out of it obviously, but you'll see it as a dot of light.
That's what's happening here.
> There's a story here about the size of the light being emitted that doesn't seem satisfactorily answered
You are misunderstanding the "size". The size "10,000" is the resolution needed to differentiate between two atoms sitting next to each other.
But in order to see the atom (detect the atom), you just need to be able to capture what it emits. And it emits photons, and it's not that hard to capture and record a single photon.
So you do, and it shows up as a single dot in the image.
If you had two atoms near each other, both emitting photons, you would not be able to distinguish them from each other (i.e. you couldn't say if there were one or two), unless they were farther apart.
Why isn't there a ruler somewhere in the picture? My brain is absolutely failing to make sense of the scales and sizes in this image.
The atom is about 255 picometers, the light is around 400 nanometers. That makes the light around 1500 times larger than the atom.
(Which also helps explain why visible light can not distinguish atoms placed close together - it's so much larger than them.)
> Why isn't there a ruler somewhere in the picture? My brain is absolutely failing to make sense of the scales and sizes in this image.
The two electrodes are about 2mm apart. About the width (not length) of a sesame seed. It's really small. The photo was taken through a microscope.
Another point of reference: the width between the electrodes is about 5,000 times the wavelength of the light being emitted.
In theory, if you had a good enough camera, the dot of light would be around 1/5000 of the width. (However the diffraction limit of your lens might enter into play, blurring the image.)
And finally, the width between the electrodes is about 8,000,000 times the size of the atom.
You can't see single atoms because they normally don't send any light toward you. But if they did, and this one is, you would see that light.
You aren't seeing the atom exactly, you are seeing what it sends toward you, and the human eye can see single photons. (And so can cameras.)
That has to be at least partially it for the rest of your comment to be right (which it is!). To say the atom was "held nearly motionless" is to say it moved a little during the exposure. It sent the light toward the sensor from all of its positions.
No, not at all. The wavelength of the light is about 1,500 times larger than the atom emitting it. So the atom would need to move at least 1,500 diameters to make any difference in the light.
It doesn't move that much.
The latter made lighting the shot in a controlled fashion a bit easier than if I had used continuous sources – you'd be looking at using a torch with a bunch of filters or a computer monitor on the lowest brightness otherwise.
Edit: based on seeing it in actual size, I would say it would be hard to see it with the naked eye, that said as pointed out in other comments, I suppose not impossible if a human eye can detect a single photon.
I'm putting together a short post with answers to some of the most commonly asked questions, but in the meantime, check out this great comment by a well-informed Redditor:
I'm assuming the two ball-pen nib shaped structures on both sides of the spec in the picture are "two metal electrodes placed about 2mm (0.078in) apart". So, the space between the left tip of the electrode and left edge of the spec is about 1 mm. Based on some "visual calculation" (zooming in the picture and doing some approximation), the spec seems to be closer to about 0.03 mm across, which is orders of magnitude larger than 438 pm that it should be. What gives?
Instead they're shining a laser at it, which excites some of its valence electrons to higher orbitals. When those electrons drop back to their ground states they emit (visible) light, some of which reaches the camera.
The atom is effectively acting as an isotropic radiator (radiating equally in all directions). The camera lens is much larger than the atom.
There are thus multiple paths from the atom to the lens and the sensor behind. Less light will reach the camera from greater angles, so the light that goes straight towards the lens or nearly straight will impact the sensor most, and that area will appear brightest. (This is my somewhat mangled attempt to explain abberation in optics...)
Even if the light were perfectly collimated in a beam the size of the atom it would still be unable to make a dot in the final image smaller than a single pixel of the sensor. If film were used instead of a digital sensor it would expose at least one pigment grain, again much larger than the atom itself.
The apparent size is an artifact of the imaging process.
You're looking at the photons emitted by an excited atom, as collected over an extremely long exposure by the camera's sensor. Which will resolve to ~1px in size... the smallest unit the camera can image.
For most of the history of the word “reflect”, its meaning was definitely not “absorb and re-emit photons of the same frequency”.
Looking at it, my suspicion is diffraction. But I don't know enough optics to be sure.
I find the parallel awesome.
Could someone explain to me how a DSLR camera can capture an image of a single atop? I thought atoms were very very small, like so small only an electron microscope could "see" them. Maybe this device has been misrepresented and it is holding a small amount of atoms rather than a single one?
 an Airy disk: https://en.wikipedia.org/wiki/Airy_disk
> When illuminated by a laser of the right blue-violet color, the atom absorbs and re-emits light particles sufficiently quickly for an ordinary camera to capture it in a long exposure photograph.
Nothing photographic, as I understand it. The atom is positively charged, and is suspended in ultra-high vacuum by electric fields. Effectively they're "weighing" it, by measuring the total charge of whatever is suspended, and knowing (from the ionization energy?) the charge on one atom.
It should be noted that this is similar to how Millikan first measured the electron charge back in 1909.
Heck, you can line up ions in a row and take pictures of those if you'd like.
The human eye after all can see a single photon. A single atom can emit a single photon, so it's not such a stretch.
My guess would be, as this is an electromagnetic trap, that two such ions would repel each other and the well is set to be shallow enough that at most one ion can remain inside.
Normally, nuclear transitions are invisibly-high energies. X-ray and gamma. But a couple of nuclei have multi-step spin isomer decays (or was it shape isomer?). One step of which is visible. So someone trapped and fully stripped a nucleus, and bombarded it to visibility. Naked-eye visibility. And took a picture. Of vacuum vessel window, with a green(?) fluorescence dot.
I saw that picture years ago. Likely a cover photo on something.
I would very much like to find it again. For use in educational content. A lot of time has been spent looking for it, both by myself, and a couple of MIT science librarians. If anyone has a clue, I'd very much appreciate it. Here is a mockup I gimped for user testing.
Why use it in education? To make the concept of atoms more concrete. Atomic electrons are too small (and slow and fragile) to see with your naked eye. But (a couple of) nuclei you can (under hard-to-contrive conditions). Absent concrete, students build their understanding of materials on muddy misconceptions. For example, one failure-mode in teaching high-school stoichiometry, is students not thinking of atoms as real physical objects.
And to preempt a common response... one professor complained "you aren't really seeing the nucleus - it's only a diffraction dot"... right before they headed out to a star party, to apparently "not see" stars. ;)
People can sense single photons: Experiment suggests that humans are capable of perceiving even the feeblest flash of light 
A scanning electron microscope, or an x-ray crystallography based machine, uses items with much smaller wavelengths, .01 - 10 nm in the case of x-rays and so they're able to peer inside an atomic structure.
Obviously the technology and intent is impressive, but will we ever be able to see an atom more clearly or is it simply too small to be seen without Photoshopping in the details?
Atoms are much more nebulous than the concept of small indivisible balls of matter.
The problem is that of scale: visible light has a wavelength of 380-750 nm (10^-9 meters) while the atomic radius of even large atoms are still in the ~200 pm (10^-12m) which means that visible light still has a wavelength 1000 times that of an atomic radius. To peer more closely at an atom, you would need to be able to use something moving at a much smaller wavelength, like an x-ray or a focused beam of electrons.
That's what I was expecting, when I read the title description: Picture of a Single Atom Wins Science Photo Contest
Like, the helical structure of DNA. I want to see how an atom looks like in reality, on the atomic scale. Along with the protons, neutrons, and maybe the electrons flying around it.
Zzz.. going back to sleep now..
That is not how reality works. Stuff is quantum mechanical and has no analog in the macroscopic world.
All one can ever see are consequences of interactions. This is what we are seeing.