Learning Object

Converging Lens with Variable Focal Length

This interactive animation demonstrates how the focal length of a lens changes with its thickness. Like in the previous slide, the lens is shown from the side, and light rays are coming in from the left. Click on the arrows on the lens and pull to make the lens longer and thinner, or squeeze it to make it shorter and thicker. Then observe what happens to the focal point. Keep playing with the animation for a little bit. This squeezable lens in the animation is actually a model for the lens in your eye, which is surrounded by a muscle, called the ciliary muscle. When this muscle contracts, it squeezes the lens of the eye, which is made of elastic fibers: The lens gets thicker. You should now be able to answer the question: How does the focal length of the eye change when the ciliary muscle contracts? – We have seen how a convex lens bundles the light rays that are coming in from very far away. But a lens can do much more than that: It can actually produce an image of a bright object!