The Eye's Distance Accomodation

So far, we have only been considering objects that are very far away from the eye. But your eyes are also capable of seeing things in focus that are closer by. How can that be? In the interactive animation on slide 10, you could verify what happens to the image when the object moves closer to the lens: the image becomes larger, and moves further away from the focal point. We cannot move the retina further away from the lens, so there is only one solution to the problem: We have to change the focal length of the cornea/lens system, and move the focal point inside the eyeball. How can this be achieved? The cornea cannot change, but fortunately, as we have pointed out already a couple of times, the eye's lens can be squeezed by the ciliary muscle, and thereby made thicker.

In this interactive animation, you can explore how the lens of the eye adjusts its thickness. This allows you to see an object in focus as it is moved to different distances from the lens. Drag the object to another position and observe how the lens changes. When you are looking at a very far away object, the lens of your eye is as thin as it can get, that means the ciliary muscle around your lens is relaxed. When you want to look at a closer object, your brain signals to the ciliary muscle to contract. This makes the lens thicker, which means its focal length becomes shorter. This way, the image is always located at the retina, and you see things in focus, no matter whether they are far or close. Keep playing with the animation and observe what happens! Try to answer the following questions: When you bring the object from far away closer, what happens to the size of the image? To the position of the image? To the shape of the lens? To the focal points of the lens?

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This completes our online optics tutorial on the eye. We hope you enjoyed working through it! And remember: ALWAYS STAY FOCUSED!!!