Flashcards in refractive errors Deck (7)
myopia or nearsightedness.
When the eyeball is too long, as shown in the drawing below, the light entering the eye ends up focusing before it gets to the retina.
Farsightedness, or hyperopia
occurs when the eyeball is too short. As a result, light ends up focusing behind the retina.
Astigmatism is different from nearsightedness and farsightedness because it makes vision blurry at all distances. To understand why, picture a corneal surface that’s not round like a marble. Instead, the surface is curved like a grape or an olive—that is, more curved in one direction than in the direction perpendicular to it.
a problem that affects people's ability to focus up close. This problem usually sets in between the ages of 35 and 45, when the lens becomes less elastic and can no longer focus near objects onto the retina. As you saw in your reading, some researchers think presbyopia may also stem from a weakening of the smooth muscles that help the lens change shape.
This is the term we use for a person who has no refractive error. This word, too, comes from the Greek—this time from a root meaning well-proportioned.
When a person is emmetropic, the eye and cornea are perfectly spherical, and the rays of light that enter the eye focus directly on the retina without any eyeglass lenses or contact lenses in front of the eye. There are some very fortunate people who stay emmetropic their whole lives until they develop presbyopia and need reading glasses.
Anisometropia is the term we use to describe people whose eyes have different refractive powers. The first part of this word, aniso-, means unequal. As you learned in today’s reading, anisometropia creates problems with vision and can also be a challenge for the optician since the glasses lenses of a person with this problem will be of different thicknesses.