Asked by: Fred Hollis, by email
Your naked eye can see objects of any size, if they emit or scatter enough light to trigger its detector cells. Light visible from the star Deneb covers a minuscule fraction of your visual field (its ‘angular diameter’ is 0.0024 arcseconds). A light-emitting object seen as the same size when 15cm from your face, would be 1.75 nanometres wide. That’s only about 10 times the width of an atom of gold! And you can ‘see’ smoke and fog, even when their constituent particles are too small to pick out.
What is limited is the eye’s resolution: how close two objects can become before they blur into one. At absolute best, humans can resolve two lines about 0.01 degrees apart: a 0.026mm gap, 15cm from your face. In practice, objects 0.04mm wide (the width of a fine human hair) are just distinguishable by good eyes, objects 0.02mm wide are not.