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Lens of the Eye: Definition & Function

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  • 0:01 What is a Lens?
  • 0:49 How the Eye Forms Images
  • 1:45 Accommodation of the Lens
  • 2:29 Focusing Disorders of the Lens
  • 3:41 Cataracts
  • 4:35 Lesson Summary
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Lesson Transcript
Instructor: Betsy Chesnutt

Betsy teaches college physics, biology, and engineering and has a Ph.D. in Biomedical Engineering

The lens of the eye focuses light and allows you to see. In this lesson, you will learn what the lens is made of and how it changes shape so that you can have clear vision.

What is a Lens?

A lens is a transparent object designed to bend light in order to form an image. Lenses are found in cameras, microscopes, telescopes, eyeglasses, and also in the eyes of humans and other animals. A lens causes the light rays that hit it to bend, and the direction the rays bend depends on the shape of the lens.

A concave lens is thinner in the middle and thicker near the edges, and it causes light rays that pass through it to spread apart. A convex lens is thicker in the middle and thinner near the edges, and it causes the light rays that pass through it to bend so that they become closer together and ultimately focus on a point other side of the lens. Because convex lenses cause light to focus, or converge, they are also called converging lenses.

This concave lens causes light rays that pass through it to be spread farther apart (shown by the white arrow)
concave lens

This convex lens causes the light rays that pass through to converge at a single point (shown by the white arrow)
concave lens

How the Eye Form Images?

The human eye is a very complicated organ, but only a few structures in the eye are important for forming images of the objects that we look at. The cornea is a thin, clear membrane that covers the outer part of the front of the eyeball. It is made primarily of a protein called collagen, which is very tough and strong. As light passes into the eye, it first passes through the cornea. Since the cornea is a curved surface, it acts like a convex lens and begins to focus the light rays.

The light then passes through the pupil and hits the lens of the eye. The lens, also convex, further focuses the light so that it will hit the retina at the back of the eyeball. The retina contains specialized cells that are sensitive to light; these are called rods and cones. When the cornea and lens focus light onto the retina, the cells are stimulated and send signals to the brain, allowing you to see!

As light rays passes though the cornea and lens, they bend so that they will focus exactly on the retina in the back of the eye. This allows you to form a clear image of the world around you.
human eye

Accommodation of the Lens

In the eye, the lens is held in place by tiny ligaments connected to the ciliary muscles. These muscles control the level of tension in the ligaments and therefore control the shape of the lens. When the eye is relaxed, tension in the ligaments causes the lens to be slightly flattened. When the eye focuses on a nearby object nearby object, the ciliary muscles contract, to reduce the tension in the ligaments and cause the lens to become more spherical. As the lens changes shape, it causes the light that passes through it to focus at a different location. This is called accommodation and is what allows your eyes to focus on both near and far objects. By contracting or relaxing the ciliary muscles, you can cause your eyes to focus on an object that is any distance away.

To focus on an object that is far away, the ciliary muscles relax and the lens gets thinner. To focus on a nearby object, the ciliary muscles contract and the lens gets thicker. This is called accommodation.
eye accommodation

Focusing Disorders of the Lens

The lens is subject to several possible problems that can prevent it from focusing properly. In many people, the lens and cornea do not properly focus light onto the retina, and this causes difficulty in focusing and blurry vision. Myopia is a condition that causes objects that are far away to appear blurry and is commonly referred to as nearsightedness. Myopia is caused when light focuses in front of the retina. It can be corrected by placing a concave lens in front of the eye (either in the form of eyeglasses or contact lenses) that causes the light to spread out before it enters the eye.

Hyperopia occurs when light focuses behind the retina and causes objects that are nearby to appear blurry, a condition also called farsightedness. It is corrected by placing a convex lens in front of the eye to cause the light to begin to converge before it enters the eye.

Presbyobia occurs as people age and the lens begins to lose elasticity so that it can no longer change shape as easily. This causes difficulty in focusing on nearby objects as well and can also be corrected with a convex lens.

Myopia causes light to focus in front of the retina and results in difficulty seeing far away objects clearly. Hyperopia causes light to focus behind the retain and results in difficulty seeing nearby objects clearly.
eye disorders

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