What Is the Smallest Object the Human Eye Can See?

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.

Naked Eye

• 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.

Telescopic Eye

• 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).