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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.
Rating Newest Oldest. Best Answer: The average adult human eye measures roughly one inch (2.5mm) in length, and is roughly spherical. This is approximately *two-thirds* the size of a ping pong ball. The eye grows about 6 mm in length from infancy to adulthood. Source(s):