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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 sight is 20/20. The first number is the distance (20 feet) from the eye chart. The second number is the distance from which a normal eye sees a letter on the chart clearly. Someone with a visual acuity of 20/20 can see certain sized letters at a distance of 20 feet. Someone with a visual acuity of 20/40 only sees letters at 20 feet that a normal eye is able to see at 40 feet.