For centuries, scientists focused on the big things in the sky: the solar system and its planets. But today, scientists also pay attention to the small organisms around us. Some of these organisms we can see with the unaided eye; others we need the help of mechanical devices to see.
How the Eye Works
As light passes through the eye's pupil (the black part of your eye, which is really a hole), it lands on the retina where photoreceptors convert the light to electrochemical signals. This allows your brain to interpret the images your eye receives.
Types of Vision
Every person's vision is most commonly graded by the Snell chart, which is the familiar board with letters that begins with a giant “E”. A person with 20/20 vision can see the letters at 20 feet that a normal person would see at 20 feet. Similarly, a person with 20/40 can see at 20 feet what a normal person would see at 40 feet.
According to the University of Utah, a naked eye (an eye without any mechanical assistance that has 20/20 vision) can see objects about 0.1 mm long. Under the right conditions, this means the naked eye could possibly see an amoeba.
With the help of a light (“normal”) microscope, the human eye can see objects at about 500 nm. For example, you can see the structures of a cell with a light microscope. Electron microscopes can see even smaller than 500 nm, including individual atoms.
Tiny Things the Naked Eye Can See
According to the University of Utah, the naked eye is able to see both a human egg and paramecium (single-cell organism). The naked eye can also see lice and nits (lice eggs).