Nearsightedness
Or Myopia refers
to an eye that can focus only on nearby objects. The far point is not infinity
but some shorter distance, so distant objects are not seen clearly. This
is caused by an eyeball that is too long or sometimes the curvature of
the corn ea is too great. Images of distant objects are focused in front
of the retina. A diverging lens causes parallel rays to diverge, allowing
the rays to be focused and correcting this defect.
digram
Farsightedness
Or Hyperopia
this is an eye that cannot focus on nearby objects. Although distant objects
are usually seen clearly, the near point is somewhat greater then the "normal"
25cm, which makes reading difficult. This is caused by an eyeball that
is too s hort or by a cornea that is not sufficiently curved. This is corrected
by a converging lens.
diagram
Astigmatism
This is usually
caused by an out-of-round cornea or lens, so that point objects are focused
as short lines. An astigmatic eye focuses rays in a vertical plane, such
as at a shorter distance than it does for rays in a horizontal plane. Astigmatism
i s corrected with the use of a compensating cylindrical lens. Lenses for
eyes that are nearsighted or farsighted as well as astigmatic are ground
with superimposed spherical and cylindrical surfaces, so that the radius
of curvature of the correcting lens i s different in different planes.