Final answer:
A cataract causes the cloudiness of the eye's lens, leading to diffused light and decreased vision. An intraocular lens implant can restore distant vision but may still require glasses for reading. The power of the intraocular lens depends on the previous condition of nearsightedness or farsightedness.
Step-by-step explanation:
The principal change that occurs in a lens affected by cataracts is the cloudiness of the lens which leads to decreased vision. When light enters an eye with a cataract, it is diffused rather than being properly focused on the retina. This diffusion is due to the clouded lens scattering the light, which reduces the vision quality. For a person with cataracts, an intraocular lens (IOL) may be surgically implanted after removing the clouded lens. This IOL is typically chosen to give the person perfect distant vision, however, because IOLs lack the dynamic focusing ability of natural lenses, the person may still require glasses for reading or seeing close objects. This is similar to presbyopia, which is caused by a loss of flexibility in the lens with age, leading to a need for a converging lens to focus on near objects. If the individual was previously nearsighted, the power of the IOL would be less than the removed lens to compensate for the elongation of the eyeball.