1. OPTICS AND THE EYE The Eye Parts of the Eye How the Eye Works (Normal Vision) Nearsightedness Farsightedness

2. INTRODUCTION §The eye is an amazing combination of physics and biology. Only by understanding image production and nerves can we truly understand what we see.

3. The eye is made up of 5 major parts. Light enters through the clear covering of the eye called the cornea. It then passes through the adjustable opening in the eye called the iris. (the colored part) The lens, which is a bit soft and flexible, is supposed to focus the image of the object onto the retina. The retina is like the movie screen of the eye. Any image formed on the retina should cause nerves to fire sending a signal along the optic nerve to be "seen" by the mind.

4. §Cornea l The clear covering of the eye. If the cornea is mis-shaped, it may lead to vision trouble. §Iris l The colored opening in the eye. The iris' job is to open or close to adjust the amount of light that actually enters the eye. §Lens l The lens is a converging lens so that it can create a real image on the back wall of the eye. The lens is actually soft in the middle (like a bag of water) so that the muscles around it can adjust its shape and focus. This allows us to focus on something close to us and far away. §Retina l The projection screen at the back of the eye. If the image gets focused here, vision is good. §Optic Nerve l The nerve that carries the signal from the retina to the brain. It is also the cause of the blind spot you have in each eye. PARTS OF THE EYE

5. THE NORMAL EYE As with any image being formed, it takes two or more rays from the same spot on an object to cross or appear to cross. In this case, looking at two rays from the candle flame, we find that the rays cross right at the location of the retina. This image would be in focus. It is important to notice that the real image of the candle is actually upside down on the retina. Your brain actually takes care of flipping it so it seems to be right side up. Some experiments that have been done involve people wearing some glasses that make everything seem upside down. After a few days, their brain flips the image. If they take the glasses off, their brain, after a delay, will flip it right side up again.

6. If the lens in the eye is too strong, or the cornea is too thick, or the eye is too long, it will cause the real image to get formed in front of the retina. This will cause a blurred image. We would call it nearsightedness (myopia). Uncorrected, nearsightedness basically means you can't see things from a distance. NEARSIGHTEDNESS

7. Since the problem is that the rays are converging too soon, we want to undo some of the convergence. Divergence undoes convergence. As a result, nearsightedness is usually corrected by placing a diverging lens in the front eye. NEARSIGHTEDNESS

8. FARSIGHTEDNESS If the lens in the eye is too weak, or the cornea is too thin, or the eye is too short, it will cause the real image to get formed in beyond the retina. This will cause a blurred image. We would call it farsightedness (hyperopia). Uncorrected, farsightedness basically means you can't see things close up.

9. FARSIGHTEDNESS Since the problem is that the rays aren't converging soon enough, we need to increase the convergence. Farsightedness is usually corrected by placing a converging lens in the front eye.