Presbyopic Eye Physics

In these photos, I am going to explain both the anatomy and the physics behind the human eye and what allows us to see the way we do. The physics portion of the eye will apply mainly to the lens of the aqueous humor. These portions of the eye are going to be responsible for refracting light into the eye and focusing it on the fovea centralis. I am going to briefly discuss the way the eye absorbs excess light so that it is not reflected out of the eye. Finally, I will discuss abnormalities of the eye and how that affects the way light is refracted. First, I am going to talk about how the lens works to allow us to see objects at different distances clearly. If you refer to figure 3, you will see the ciliary zonules attached to the lens via

The most prevalent concern in today’s society is the need for reading glasses due to farsightedness. When this occurs in early life it is known as a hyperopic eye, but when it occurs due to age it is known as presbyopia which literally translates to, “old vision”. These may seem to have the same effect but the reasoning behind them can actually be quite different. In the case of hyperopic eye, you eyeball itself is too short and your lens adjusts the focal point to fit the dimensions of a normal eye. This causes the image to form after the retina. In the case of presbyopia, the lens becomes hard due to age and makes it difficult for the ciliary muscles to contract. Since you cannot make the lens wider to accommodate for the shorter object distance the same thing occurs as stated in the hyperopic eye. Both of these problems can be corrected with a convex lens. Another less common problem can occur when the eyeball is too long, otherwise known as a myopic eye. In this case, the image forms before it reaches the retina and the light rays begin to spread back out. This type of vision can be corrected with a concave lens. All of these eye pathologies can be seen in figure 4 above. The final eye pathology I am going to talk about is called an astigmatism. When this occurs it basically means that either the cornea or lens has some sort of imperfection. It usually

The most widely accepted theory of how the lens and ciliary muscles bend light is called the Helmholtz theory and was proposed in 1885. This theory says that when the ciliary muscle contracts, it causes the muscle to move inward towards the lens and the ciliary zonules lose tension, which would widen the lens. Most people have accepted this theory for years and continue to accept this theory; however, a new theory has recently been proposed in 1992. This new idea is called the Schachar mechanism and it states that essentially the opposite happens. This theory states that when the ciliary muscles contract it puts more tension on the lens and flattens it, instead of widening it. It also claims that when the ciliary muscles are relaxed there is less tension on the ciliary zonules which widens the lens. Scientists have been debating between the two for more than 20 years now and still have not conclusively decided on one or the other. I personally believe that the first theory is correct because it shows why our eye prefers to focus on distant objects. If the flatter lens has a focal point of 20mm, which is what our eye wants, then we would want to keep it at this focal point value with minimal effort. The second theory proposes that we need to contract the ciliary muscle in order to keep the lens flat, which would not be the most efficient