If so, what are some examples? If not, what is the smallest thing our eyes can see?
As with all questions of this sort, you will get better and more helpful answers if you research the question a bit yourself first. Google for "smallest object we can see", then come back here with any more specific questions you still have.
To see something requires light to be reflected by it. Light is reflected by causing charges to oscillate.
I would say that you cannot reflect light off of anything smaller than an electron, because I think that an electron is the smallest charged particle.
It's worth noting that visible light has a range of wavelengths of about 400 to 900nm. An object that is much smaller than one wavelength will not reflect or obstruct / scatter much of the light because of the diffraction pattern. Seeing would be progressively harder an harder but there is not a simple 'cut-off' As has already been written, it would be best to read around before pursuing this thread further.
With or without a microscope or other optical/electronic aid ??
there's a huge difference
Optical instruments can help but there are fundamental limitations. Which is where the Electron Microscope comes in handy.
yup which was why I said optical/electronic ;)
The OP's Q was very vague, just trying to find out what he / she really meant
No response from the OP yet. Perhaps we should wait, instead of talking amongst ourselves. :)
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