40 microns
In practice, particles smaller than 325 mesh are usually described in microns. A micron is one thousandth of a millimeter, or one millionth of a meter. The unaided human eye can see particles of about 40 microns. Smaller than that, you need magnification. 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). Very nice question! First of all, the 'smallest size' that a human eye can perceive is called visual acuity, and can be expressed in various ways. It cannot simply be expressed by means of size measures, as objects with a fixed size are perceived as smaller when viewed from a distance (perspective). A 20 micron particle is not visible with the naked eye. 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. First of all, the 'smallest size' that a human eye can perceive is called visual acuity, and can be expressed in various ways. 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. So, I wondered, what is the smallest size a human eye can actually see? According to a whole bunch of sources all across the internet, it's either 200-400µm, 100µm, or 58-75µm. I also hear 'the width of a human hair' very often, but those can range from 17 to 181µm.
http://www.skylighter.com/fireworks/how-to/Measure-particle-size.asp http://www.sciencefocus.com/qa/how-small-can-naked-eye-see http://biology.stackexchange.com/questions/26189/whats-the-smallest-size-a-human-eye-can-see http://www.answers.com/Q/Is_a_twenty_micron_particle_visible_with_naked_eye http://biology.stackexchange.com/questions/26189/whats-the-smallest-size-a-human-eye-can-see http://www.sciencefocus.com/qa/how-small-can-naked-eye-see http://biology.stackexchange.com/questions/26189/whats-the-smallest-size-a-human-eye-can-see
what is smallest particle size a human eye can see