talk about eye muscle

  Continue the previous post, do you know why having two pair of glasses is good? Let have a look at the eye structure.

 For person who has short-sighted. Image is focused in front of the retina. Glasses with concave lens can make the light beam diverse and the image can then be focused on the retina and become clear.

  How do the eyes regulate the thickness of the eye lens? There are ligaments which are group of fibers linking with the eye lens. It is not hard to imagine when the fibers are tensed up, the lens will be pulled and become thin. A thinner lens results in longer focal length and distinct object can be focused.On the contrary, when the fibers are relaxed and loose, the lens will not be pulled and become thicker.A thicker lens has shorter focal length and close object can be focused.

  Then, what's the relations between ciliary muscle and the ligaments?I must warn you it is not as obvious as you may expect. I have pretended to understand and attempted to explain. But after a while, to be honest, I still have no idea but just memorizing the fact. I memorized this since I have been in high school: "Contracting the ciliary muscle will let the ligaments relax and the eye lens will become thick." The opposite surely holds. Do you sense something weird? Try to imagine a string is tied on a pillar and your hand is holding the other end. When you contract your arm muscle and pull the string, the string will then be tensed. When you release the string, then the string will be loose. Hand is in analogous to ciliary muscle and string is for ligament. But the fact about ciliary muscle is completely opposite!

  Back to the story, why should we prepare two pair of glasses? First of all, short-sighted glasses are designed for seeing distinct objects clearly. But when you wear glasses to see close object, the concave lens in glasses are overkill. It let the light beam diverse too much and therefore the ciliary muscle should contract more to let the eye lens thicker.Contraction needs force and energy and you will feel tired. Therefore, if you wish to see close object, let wear another glasses with lighter degree(less concave) and the light beam will not diverse too much. It results in thinner eye lens, less eye muscle contraction and you will feel less tired.