We know that the objects are made up of atoms. We also know that we cannot see atoms with the help of light as the wavelength of light is too big in comparison to atom. So, my question is then: how can we see objects with the help of light if we cannot see atoms?
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2$\begingroup$ Related: How can we see an atom now? What was the scale of this equipment? $\endgroup$– Emilio PisantyCommented Aug 1, 2020 at 15:55
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1$\begingroup$ Related: How many atoms does it take for us to perceive colour?. $\endgroup$– user21820Commented Aug 2, 2020 at 14:54
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$\begingroup$ Why do you think you can't use light to see a single atom? popularmechanics.com/science/a17804899/… $\endgroup$– Eric TowersCommented Aug 2, 2020 at 20:19
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5 Answers
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As explained in a related previous thread, the human eye cannot resolve atoms (in the sense that visible light is too coarse to distinguish one atom from its neighbours in solids and liquids), but that does not mean that we cannot perceive the light that they emit.
The light emitted by each atom is generally quite faint (probably just under the detection threshold for human vision), but if the atom is emitting (or reflecting, or scattering) photons, there's a nonzero probability that the cells in the human eye will detect it.
If you then put a bunch of atoms together, they will emit more photons, and the probability that the cells in the eye will detect it increases. Put enough atoms together, and it becomes certain that the eye will detect the signal. We cannot tell with certainty where the signal comes from to any accuracy better than the wavelength of light, but that does not mean we cannot detect the signal.
answered Aug 1, 2020 at 16:00
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This can probably be answered by analogy. If you look at an individual pixel on a TV screen it doesn't tell you much about what picture is on the screen. However, if you have a screen with several million pixels, when viewed together you are able to process them into a meaningful image.
The situation is similar with atoms. Your eyes do detect individual photons coming from atoms, however just like looking at a single pixel, seeing a single photon from a single atom would not even register on your vision.
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6$\begingroup$ Note that the human eye is apparently sensitive to individual photons. That sensitivity manifests more as a "slightly better than 50% chance at predicting whether a photon was fired" than as vision, but it's there. This answer also doesn't really address how light can interact with the atoms of an object when the light is "bigger" than the atoms. I believe an important part of the situation is that while a single atom can only interact with very specific photons, a photon can induce and be affected by collective excitations in a bulk of atoms. $\endgroup$– HTNWCommented Aug 1, 2020 at 22:20
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3$\begingroup$ Most of the time, I don't think you are seeing photons coming from individual atoms, you're seeing photons reflected from electromagnetic fields in the material. You'd see emitted photons if they're created by the material, as with LEDS, or absorbed and re-emitted, as with fluorescence. $\endgroup$– jamesqfCommented Aug 1, 2020 at 23:25
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$\begingroup$ @HTNW/jamesqj Thanks for clarifying, I'm not overly familiar with the mechanics of the eye. $\endgroup$– CharlieCommented Aug 2, 2020 at 12:08
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$\begingroup$ @Charlie: Nothing to do with the eye, really, since at that point a photon is just a photon. It's more about the mechanism of reflection from surfaces, which accounts for most of the photons we see. While I'm certainly no expert, I seem recall that reflection is basically a bulk property. $\endgroup$– jamesqfCommented Aug 2, 2020 at 16:02
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It depends with what you mean with the word "seeing", because seems to me that you are confusing it with the word "resolve".
You cannot resolve the particulars of the atomic structure just with electromagnetic radiation, because higher wavelenght as you said are "too big" for the atoms; the human visible light has a wavelenght not too less than $1\mu$m, while the extension of a whole atom is in general around $0.1$nm. Theoretically you can go with higher frequency radiation, so less wavelenght, like X-rays or gamma radiation, but Nature is smarter and what happens with this energy scale of radiation is that it starts to actively interact with atomic structure and so, even in this case, you have a limit in your ability to resolve atomic structure.
But even if you have the most precise instrument in the world quantum mechanics states that the basic principle of Nature is indetermination, so it's very hard to say what you would actually "see", maybe like a substantially blurred picture if you just want to imagine it.
So in conclusion light interacts with atoms with absorption, reflection and diffraction, that's what allows you to perceive trough vision complex bodies; but at the same time, given the underteministic nature of the things in the World, doesn't allow you to resolve them with arbitrary precision.
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$\begingroup$ It's actually possible to "see" and manipulate individual atoms with considerable precision, e.g. IBM's classic demonstration of their logo with xenon atoms: en.wikipedia.org/wiki/IBM_(atoms) $\endgroup$– jamesqfCommented Aug 2, 2020 at 16:08
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$\begingroup$ Sure, I was talking about resolving the intimate structure; that's substantially not possible $\endgroup$– Rob TanCommented Aug 2, 2020 at 17:31
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$\begingroup$ The part with the X-rays is exactly what they are doing with the free electron lasers like the European XFEL en.wikipedia.org/wiki/European_XFEL. $\endgroup$ Commented Aug 3, 2020 at 11:23
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Imagine kicking many footballs at a large pile of invisible bricks. The fact that the footballs rebound tells you that the pile is there, and the angles at which they rebound tells you something about the overall shape of the pile, but you won’t be able to work out the position of individual bricks in the pile.
answered Aug 1, 2020 at 15:20
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$\begingroup$ Unless of course, you get very good at kicking the balls with high precision and accuracy from very close range. $\endgroup$– NijCommented Aug 2, 2020 at 8:32
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1$\begingroup$ @Nij And you use footballs with a small enough <s>wavelength</s> diameter. $\endgroup$ Commented Aug 2, 2020 at 14:25
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how can we see objects with the help of light if we cannot see atoms?
It is easy to think of a solid material as "just a bunch of atoms", but for photons (which interact with charges), that isn't the case.
Solid materials are made up of molecules, or large numbers of bonded atoms. When bonded, the positions of the (charged) electrons changes from a very localized position near the nucleus of the atom to a smeared-out surface surrounding the molecule or bulk material.
The visible light reaching this surface has a much higher chance of interacting. Given a bulk material of some thickness, the chance goes way up for it to interact with at least some of the material. For a material with homogenous properties from one region to another, all these different interactions give similar results. So we can sum the light that is reflected in a meaningful way.
To image atoms, we need the light to interact with one specific atom. Detecting the interaction with other nearby atoms does not help us image this one. The low likelihood of a visible photon interacting with an unbound atom makes this task too noisy to be useful.
