Correcting Vision Problems
We have seen in the last video how we can shift the focal point if we add a second lens to the original convex lens that is the model for our eye. This interactive animation lets you explore how to correct the vision problems of near sightedness and far sightedness. You can select either a near sighted eye, for which the eyeball is too long, or a far sighted eye, which has too short of an eyeball. You can then choose the appropriate type of eye glasses that will move the focal point onto the retina. For comparison, there is also a normal sighted eye for which the focal point is located on the retina. This eye sees distant object in focus, without the help of glasses. Play with the different arrangements, and describe what happens when you combine each of the eyes with a convex and with a concave lens. Make sure you understand which correction lens belongs to which vision problem, and why. Note how the concave lens in front of the eye spreads out the incoming rays, such that after passing through the eye's lens, they are not focused as close to the lens as before. The convex lens in front of the eye does just the opposite: now the focal point is closer to the lens than without it. And here is a tricky question: How come far sighted people can actually see far away objects just fine in focus, without eye glasses? Remember that the lens can be squeezed – and so far, we have looked at the eye only with the lens relaxed, that is, with the lens as thin as it can get.