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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.
Under such typical dark sky conditions, the naked eye can see stars with an apparent magnitude up to +6 m. Under perfect dark sky conditions where all light pollution is absent, stars as faint as +8 m might be visible. The angular resolution of the naked eye is about 1â²; however, some people have sharper vision than that.