a piece of glass or other transparent material so shaped as to converge or scatter light rays. See also glasses
the transparent, biconvex body separating the posterior chamber and the vitreous body of the eye; it refracts (bends) light rays so that they are focused on the retina
. Called also crystalline lens
. In order for the eye to see objects close at hand, light rays from the objects must be bent more sharply to bring them to focus on the retina; light rays from distant objects require much less refraction
. It is the function of the lens to do accommodation
, making of adjustments for viewing both near objects and more distant ones. To accomplish this it must be highly elastic so that its shape can be changed and made more or less convex
. The more convex the lens, the greater the refraction
. Small ciliary muscles create tension on the lens, making it less convex; as the tension is relaxed the lens becomes more spherical in shape and hence more convex.
With increasing age the lenses lose their elasticity; thus their ability to focus light rays in the retina becomes impaired. This condition is called presbyopia
. In farsightedness (hyperopia
) the image is focused behind the retina because the refractive power of the lens is too weak or the eyeball axis is too short. Nearsightedness (myopia
) occurs when the refractive power of the lens is too strong or the eyeball is too long, so that the image is focused in front of the retina.
The biconvex lens of the eye. From Frazier et al., 1996.
achromatic lens one corrected for chromatic (color) aberration.
apochromatic lens one corrected for chromatic (color) and spheric aberration.
biconcave lens one concave on both faces.
biconvex lens one convex on both faces.
one having two segments with different refracting power, the upper for far vision and the lower for near vision. See also bifocal glasses
one curved like a section of the interior of a hollow sphere; it disperses light rays. Called also diverging lens
contact l's corrective lenses that fit directly over the cornea of the eye; see also contact lenses.
converging lens (convex lens) one curved like the exterior of a hollow sphere; it brings light to a focus.
convexoconcave lens one that has one convex and one concave face.
crystalline lens lens
one with at least one nonspherical surface, used to correct astigmatism
honeybee lens a magnifying eyeglass lens designed to resemble the multifaceted eye of the honeybee. It consists of three or six small telescopes mounted in the upper portion, directed toward the center and right and left visual fields. Prisms are included to provide a continuous, unbroken magnified field of view.
one whose power increases continuously and regularly in a downward direction, avoiding the discontinuity in field and power inherent in bifocal
and trifocal lenses
orthoscopic lens one that gives a flat and undistorted field of vision, especially at the periphery.
planoconcave lens a lens with one plane and one concave side.
planoconvex lens a lens with one plane and one convex side.
an apparatus used in the diagnosis of astigmatism
trial l's ones used in testing the vision.
trifocal lens one having three segments of different refracting powers, the upper for distant, the middle for intermediate, and the lower for near vision.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.