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The smallest particle visible to the naked eye is 40 microns. To give you some perspective, a piece of hair's width can range from 40 to 120 microns.
The normal pupil size of a human eye is 4mm, which sets a minimum angular resolution of the eye at [math]2\times10^{-4}[/math]rad. Obviously, we want to put small objects as close to our eyes as possible to be able to see them, but there is a minimum distance for comfortable viewing which is roughly at 25cm. This works out to a size of 0.04mm if the human eye is diffraction-limited, but unfortunately, our eyes do not work at this higher limit.