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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.
Cell Size and Scale. Notes. Some cells are visible to the unaided eye. The smallest objects that the unaided human eye can see are about 0.1 mm long. That means that under the right conditions, you might be able to see an ameoba proteus, a human egg, and a paramecium without using magnification. A magnifying glass can help you to see them more clearly, but they will still look tiny.