What do we see if we look at a molecule or atom through an optical microscope?
All people, and even more so schoolchildren, are familiar with pictures like the one above. But would we see something similar if we looked at a molecule or atom through even the coolest optical microscope?
We know that there are optical instruments such as microscopes. These devices allow us to look at very small objects. You can see the cells of living organisms and even their internal structure, as well as many other things that are not visible to the normal eye. But can we go one step further and see the molecules that make up cells?
Strictly speaking, all microscopes have a certain limit to which they can magnify a picture, or in other words, how small an object can be seen through them. And let’s face it, there is no optical microscope that can view an object as small as a single molecule or atom. Some fundamental laws of optics prevent this.
But suppose, however, that we were able to obtain a microscope with such an awesome resolution that it would be ready to show us an object as small as a single molecule or atom. So we point this microscope at a single atom or molecule, what do we see?
The answer is very simple: virtually nothing. The fact is that light is made up of photons. And in order to see any object with a microscope or even just with an eye, and to consider its structure, we need many photons to be reflected simultaneously from different parts of this object and to fall afterwards in abundance into the eyepiece of the optical device or into our eye. Of course it is also possible that the object itself is luminous, i.e. emits photons. The meaning is the same, a lot of photons from different parts of the object must fall into the optical device simultaneously.
But molecules or atoms interact with photons differently. They don’t reflect them. Molecules and atoms can only absorb or emit photons. And more often than not, photons fly past molecules and atoms without interacting at all. An atom or molecule usually emits only one photon at a time, which then flies in an unpredictable direction. In addition, in order for a molecule or atom to emit a photon, it (or he) must first absorb another photon.
Thus, our experiment of observing a molecule with a microscope would look like this: we illuminate an atom or molecule with light, but most of the photons fly past the atom or molecule in question. Then, at some point, one of the photons is still absorbed, and after some time the atom or molecule emits a similar photon in an unpredictable direction (often, by the way, it flies in the same direction as the previously absorbed photon came from).
Thus, we can hardly distinguish this single photon emitted by a molecule or an atom from the illumination we shine on the object under study, i.e. billions of other similar photons flying nearby. And if the molecule is not illuminated, the photon it emits may have to wait a very long time.
But even if we can separate the photon emitted from the molecule from the illuminated photons, still, the best we can expect from a molecule or an atom, which we look at through an optical microscope, is that at some unpredictable moment, it (or it) will still emit one photon towards the microscope eyepiece. A single photon is not enough for the human eye to see anything. But a microscope can probably be made in such a way that it can pick up and amplify the signal received when a single photon hits it.
In any case, the best we get when observing a molecule or an atom with the coolest optical microscope is vanishingly small and rare flashes of light, lasting for an elusive moment, occurring at unpredictable moments. These flashes will in no way visually reflect the structure of a molecule or atom (although their spectrum will contain some information about it), and we will never see a picture like the one above. Generally speaking, the picture above is only a poor visualization of the internal structure of the atom, which has been investigated by other methods, but not by an optical microscope.