How Glasses andContact Lenses Work
Poor vision is commonly caused by incorrect shape of the eyeball, incorrect shape of the cornea, and hardening of the lens. Lucky these can be corrected by wearing glasses or contact lenses.
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Myopia- If you have Myopia, also known as near-sightedness, then your eyes have trouble focusing on objects in the distance. When the cornea and eye lens refract light, it brings the rays together, but by the time it reaches the retina they have begun to spread out and the image becomes blurry. You can fix this by wearing glasses or contact lenses that have a diverging lens. A diverging lens spreads out the rays before they reach the eye, making it appear as though the rays are coming from an object that is closer to the eye.
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Hyperopia- If you have Hyperopia, also known as far-sightedness, then your eyes have trouble focusing on objects nearby. The light coming from a nearby object is refracted by the cornea and eye lens, but the rays meet the retina before they meet, making the image blurry. This can be corrected by a converging lens. A converging lens bends the rays slightly, so it brings them together before they reach the cornea, the lens of the eye refracts the rays so the rays are focused on the retina.
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Presbyopia- As people get older the lenses of the eyes become stiff, and the ciliary muscles can no longer change the lenses shape. People with Presbyopia have difficulty focusing on nearby objects, but if you were already near-sighted and got Presbyopia, then you would have difficulty focusing on both far and nearby objects. People who have Presbyopia use bifocals to correct it. Bifocals lenses have two parts, the top which corrects near-sightedness, and the bottom which corrects far-sightedness.
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Astigmatism- Astigmatism is commonly caused by incorrectly shaped corneas. This distorts and blurs vision, because the cornea is oval, instead of rounded. This causes parts of images to be focussed, while the other parts become blurred. Depending on the type of astigmatism, this can be corrected with laser eye surgery, glasses or contact lenses.