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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 eye is structurally designed to detect and process light. The cornea and the crystalline lens are both important for the eye to focus light. The eye focuses light in a similar way to when you use a magnifying glass to concentrate the Sunâs rays onto a piece of paper. The distance from the magnifying lens to the piece of paper is the focal length. For the eye, light from distant objects is focused onto the retina at the back of the eye. The eye is about the size of a table tennis ball, so the focal length needs to be about 2.5 cm.