Not everyone was lucky enough to be born with 20/20 vision, which is why contact lenses and eyewear were invented. The reason that certain people don't have 20/20 vision is because light that enters their eye doesn't shine on the retina accordingly. Instead, the light is distorted in the eye before reaching the retina, preventing it from hitting a single target, which is needed for vision to focus accurately. This distortion of light by the eye can be caused by a few different physical imperfections, like a misshapen cornea, or an elongated or shortened eyeball.
Purpose of the Lens
The contact lens' purpose is to redirect the light into a single focal point on the retina, therefore restoring correct vision. Contact lenses work by fine-tuning the light into your eyes, similar to the way a magnifying glass focuses sunlight to a single point. Depending on the circumstances, many visually impaired people can receive 20/20 vision with contacts.
Inner Surface Shape
The shape of the contact lens depends on the shape of your eye and whether you are nearsighted or farsighted. The inner surface of the lens must adhere to the outer shape of you cornea, so it can stick to your eye like a suction cup. Eye doctors diagnose this shape through a serious of tests and measurements, which are inscribed on your prescription. Once the correct shape is determined, the moisture that is naturally produced by your eye acts like an adhesive for the lens, and keeps it in place.
Outer Surface Shape
Once the inner shape is determined, the shape of the contact lens' outer surface comes into play. The outer surface of the lens focuses the light that enters your eye, and it is measured in diopters. If you are nearsighted, the diopter will be a minus sign (-), if you are farsighted, it will be a plus sign (+). The degree of the diopter measurement depends on the severeness of the impaired vision. That being the case, the shape of the lens' outer surface isn't flat all the way around, it has a kind of bubble in the center. Once the contact lens is in place in your eye, it can direct light accordingly to your retina, therefore improving your vision.
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Also see: How to Make Sense of Your Contact Lenses Prescription | Dry Eye Contact Lenses | Types of Contact Lenses | The History of Contact Lenses | Disposable Contact Lenses