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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.
In broad daylight, humans can visualize objects in the glaring light from the sun, while at night large objects can be detected by starlight when the moon is dark. At threshold sensitivity, the human eye can detect the presence of about 100-150 photons of blue-green light (500 nanometers) entering the pupil. For the upper seven decades of brightness, photopic vision predominates, and it is the retinal cones that are primarily responsible for photoreception.