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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 smallest size visible through the naked eye is generally about 0.1 mm (100 Âµm) and through a light microscope is about 0.2 Âµm (200 nm); anything less than 1 Âµm cannot be seen clearly without an electron microscope. However, living organisms cannot be observed using an electron microscope.