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Comment by tjohns | Hacker News Reader

You're not looking directly at the atom itself (which is impossible... atoms are smaller than light's wavelength).

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.

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It doesn't really mean anything to day "you aren't looking at it, you're looking at the reflected photons". You're always looking at reflected photons of everything.

Actually “reflected” doesn´t mean much for photons: they are absorbed and re-emitted.

Isn't that what "reflected" means?

Not exactly.

https://en.m.wikipedia.org/wiki/Reflection_(physics)

For most of the history of the word “reflect”, its meaning was definitely not “absorb and re-emit photons of the same frequency”.

It doesn't appear that the atom is 1 pixel, but is in fact quite a bit larger than this: there are lots of elements in the image which are significantly smaller than it. So what gives?

Possibly diffraction, possibly JPEG artifacts, possibly camera movement. Tough to say without more details.

Looking at it, my suspicion is diffraction. But I don't know enough optics to be sure.

I used to work on trapped ion experiments, and the limitation of the atom size was always the diffraction limit, which is limited by the NA of the lens (f-stop in camera terms) and the wavelength of light. In this case, the optical system (I'm guessing a camera lens) is designed for multiple wavelengths, so it might not reach the diffraction limit at the emission wavelength, which is like 400nm. In that case, the limit would be the aberration of the camera lens, which can be wavelength-dependent. Most camera lenses aren't designed for 400nm light, which is marginally visible.

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