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 the Figure below. 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).
- The optic nerve carries electrical signals from the rods and cones to the brain.
Q: The lens of the eye is a convex lens. How would vision be affected if the lens of the eye was concave instead of convex?
A: A concave lens causes rays of light to diverge, or spread apart. It forms a virtual image on the same side of the lens at the object being viewed. Therefore, a concave lens would focus the image in front of the eye, not on the retina inside the eye. No signals would be sent to the brain so vision would not be possible.
How We See
The ability to see is called vision. This ability depends on more than healthy eyes. It also depends on certain parts of the brain, because the brain and eyes work together to allow us to see. The eyes collect and focus visible light. The lens and other structures of the eye work together to focus an image on the retina. The image is upside-down and reduced in size, as you can see in the Figure below. Cells in the retina change the image to electrical signals that travel to the brain through the optic nerve. The brain interprets the electrical signals as shape, color, and brightness. It also interprets the image as though it were right-side up. The brain does this automatically, so what we see always appears right-side up. The brain also interprets what we are seeing.
Q: The part of the brain that processes information from the eyes is the visual cortex. It is located at the back of the brain. How might an injury to the visual cortex affect vision?
A: An injury to the visual cortex might cause abnormal vision or even blindness regardless of how well the eyes can gather and focus light.
- The human eye is an organ that is specialized to collect light and focus images. Light enters through transparent structures at the front of the eye. It is focused by the cornea and lens on the retina at the back of the eye.
- The ability to see is called vision. The brain and eyes work together to allow us to see. The eyes collect and focus visible light and change it to electrical signals that travel to the brain. The brain interprets the signals and “tells” us what we are seeing.
- Create a flowchart to summarize how the eye gathers and focuses light.
- Explain the meaning of the following sentence: The eyes sense light but the brain sees.