Physlet by Wolfgang Christian
The applet is a simplified model of the eye in which the front of the eye is a single converging lens.
Initialize a healthy eye by clicking on the link at the bottom of the applet. Then add a far source of light using the link at the top of the applet. Notice how the parallel rays of light from the far away source converge at the back of the eye on the retina. The retina is to the eye what film is to a camera. The retina is made up of nerves that convert the light energy into an electrical signal that is sent to the brain. So in order for an object to be "seen" its image must be FOCUSED on the back of the retina.
Now remove the far source and add a near source. Notice that the light from the near by source is focused behind the retina. In this case the person would see a blurry image. As evolution would have it, our eyes have the ability to accommodate. You can change the focal length of your eye by using the muscles of your eye to change the curvature of the lens. Try looking at a far away object and then at something close by, such as your finger. You will feel the muscles in your eye respond as you change your focus. In the applet accommodation is accomplished by using the slider at the bottom to vary the focal length of the lens. Vary the focal length of the lens, using the slider, until the image of the light source is focused on the retina.
Viewing Far Away Objects
People with normal vision focus on far away objects with their eyes relaxed. Notice that the far source in the applet was focused when the focal length was at its maximum, 1 unit. As you use your muscles to accommodate you shorten the focal length of your eye.
Near Point and Far Point
Put your finger in front of your eyes about an arm's length away. You should be able to see a clear image of your finger. Now slowly bring your finger toward you. At some point, you will no longer be able to focus on your finger and it will become blurry. This is your near point. It is the closest distance at which you can focus on an object. If you have not already done so, initialize a healthy eye with a near source of light focused on the retina. Now move the source of light toward the eye. At some point you will no longer be able to accommodate (using the slider) to focus the source. That is the near point for the eye in the applet. Notice that the eye in the applet is not to scale relative to a real eye. If we had made it to scale you would need a MUCH wider computer screen.
The far point is just like the near point except it is the furthest point an eye can focus on. For people with normal vision, the far point is at infinity.
Nearsightedness and Farsightedness
Initialize a nearsighted eye and add a far source. Notice that the light does not focus on the retina when the eye is relaxed. Instead, it focuses in front of the retina. Use the slider to try and focus the light. Notice that accommodation does not help in this situation. Now remove the far source and add a near source. Notice that the nearsighted person has no trouble focusing on the near by source. A person who is nearsighted can clearly see near objects but not far away objects.
Now initialize a farsighted eye and investigate it as you did with the nearsighted eye. Notice that a farsighted person can see far away objects but has difficulty focusing on near by objects.
Initialize a nearsighted eye with a far source. Unaided, this eye can not focus on the far source. Now add an eyeglass lens. Notice that you can change the focal length (power) of the eyeglass lens by clicking on it and then dragging on the hotspots. You can make the lens either converging or diverging.
Since light is focused in front of the retina in a nearsighted eye, nearsightedness is corrected using a diverging lens. Can you find the correct focal length to correct this eye? In the same way, farsightedness is corrected using a converging lens.
Illustration by Melissa Dancy and Wolfgang Christian