http://www.ck12.org Chapter 5. Energy
5.59 Vision and the Eye
- Describe how the eye collects and focuses light.
- Explain the role of the brain in vision.
Did you ever hear the proverb, “the eyes are the windows to the soul”? While it’s true that our eyes often express
what we are thinking or feeling, they are windows in another sense as well. Like windows made of glass, the eyes
let light in and allow us to see.
Structure and Function of the Eye
The human eye is an organ that is specialized to collect light and focus images. The structures of the human eye are
shown in theFigure5.139. Examine each structure in the diagram as you read about it below.
- The sclera, also known as the white of the eye, is an opaque outer covering that protects the eye. It keeps light
out of the eye except at the center front of the eye. - The cornea is a transparent outer covering of the front of the eye. It protects the eye and also acts as a convex
lens. A convex lens is thicker in the middle than at the edges and makes rays of light converge, or meet at a
point. The shape of the cornea helps focus light that enters the eye. - The pupil is an opening in the front of the eye. It looks black because it doesn’t reflect any light. All the
light passes through it instead. The pupil controls the amount of light that enters the eye. It automatically gets
bigger or smaller to let more or less light in as needed. - The iris is the colored part of the eye. It controls the size of the pupil.
- The lens of the eye is a convex lens. It fine-tunes the focus so an image forms on the retina at the back of the
eye. Tiny muscles control the shape of the lens to focus images of close or distant objects. - The retina is a membrane lining the back of the eye. The retina has nerve cells called rods and cones that
change images to electrical signals. Rods are good at sensing dim light but can’t distinguish different colors
of light. Cones can sense colors but not dim light. There are three different types of cones. Each type senses
one of the three primary colors of light (red, green, or blue).