Question

the upper portion of bifocal lens is used for correcting

Answer 1

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.

bifocal lens one having two segments with different refracting power, the upper for far vision and the lower for near vision. See also bifocal glasses.

concave lens 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 (def. 2).

cylindrical lens one with at least one nonspherical surface, used to correct astigmatism.

diverging lens concave lens.

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.

omnifocal lens 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.

Stokes's l's 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.

bi·fo·cal lens

a lens used in cases of presbyopia, in which one portion is suited for distant vision, the other for reading and close work in general; the reading addition may be cemented to the lens, fused to the front surface, or ground in one-piece form; other bifocal lenses are the flat-top Franklin type, or blended invisible.