Background

First, it is important to realize that lenses only focus at one distance clearly. That is why, when you use a pair of binoculars, you have to ajdust the focus as you look from near to far. In the eye, we need to be able tofoxue as well; we something that can ajust its focus.

The Lens (also called the Crystalline Lens) is the adjustable focusing element of the eye. It is located just behind the iris. This process of adjusting the focus for different distances by changing the shape of the lens is called accommodation. Accommodation is the process of adjusting the lens of the eye so that you can see both near and far objects clearly. This process is very rapid although changing accommodation from a near object to a far object is faster than going from a far object to a near object (Kirchhof, 1950). Accommodation is controlled by muscles connected to the lens, called ciliary muscles. The ciliary muscles work automatically without conscious control. The ciliary muscles can contract and increase the curvature of the lens so that the lens thickens. The increased curvature of the lens allows the eye to focus on a close object. When the person then has to look at a faraway object, the muscles relax and the focus of the lens changes to an object further away. Look at an object close to you, such as the text of the book. Then look up and look out the window and across the street. As you do, the process of accommodation automatically adjusts your focus.

Use this activity to explore how accommodation helps us keep objects at different distances in focus.

Instructions

Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

Illustration Tab

Settings

Below is a list of the ways that you can alter the illustration. The settings include the following:

Light Position: moves the light closer or farther from the eye.
Turn Light On: pressing this button will start the light. Pressing the button again removes the light.
Eye Accommodates: when checked, the lens of the eye accommodates, or adjusts size, to keep the light in focus. When not selected, the eye does not accommodate and only one distance from the eye will be in focus.

Reset

Pressing this button restores the settings to their default values.

Try these settings to see how accommodation works

Eye Accommodates

The checkbox to make the eye accommodate has not been selected so the eye will focus clearly only at one distance.

On the right, you have an eye. The lens is drawn as a gray oval in this diagram. The red dot is the light source. When light leaves a light source, it spreads out. In this diagram, only the light that will enter the eye will be drawn. When light hits a lens, the light bends. Since the lens of the eye is thicker in the middle than the ends, the light bends so that the light moves back together. There is one place where all the rays will gather into one point. This is the focal point. Where that point is depends upon the distance the light is from the lens and the strength of the lens, so moving the light will change the focal point if the strength of the lens does not change. Try the diagram without a lens that accommodates. Thus, accommodation is where the lens adjusts its strength so that the focal point always stays on the retina. Next, make the eye accommodate and move the light to see how the lens adjusts to keep the light focused on the retina.

Proceed to Quiz