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Glossary NCLE Exam/145 Questions and Answers
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Glossary NCLE Exam/145Questions and Answers
Accommodation - -The ability of the crystalline lens to adjust power to
provide retinal focus of images of objects closer than optical infinity
-Aphakia - -Absence of the crystalline lens.
-Astigmatism - -A refractive defect where the refractive surface has
different power in different meridians As a result the image formed of a point
object will not be a point but a line
-Axis (cylinder) - -The weakest or least curved meridian on the surface of a
cylindrical or toric surface
-Axis (principal) - -An imaginary line going through the geometric center of
an optical system perpendicular to the surface.
-Base Curve - -Of a contact lens the curvature of the posterior optical
portion otherwise known as the Posterior Central Curve
-Cylinder - -An object whose surface is curved differently in different
meridians with the axis having zero curvature
-Cylinder Lens - -A lens with different power in different meridians due to
one or more surfaces having cylindrical or toric shape
-Cylinder Power - -The algebraic difference in power between the principal
meridians of a cylinder lens
-Diopter - -A unit of measurement equal to the reciprocal meter used to
measure vergence of light and power of optical lenses
-Dispersion - -The separation of white light into its spectral colors
-Effectivity - -Changes in the vergence of light between one point and
another In contact lens practice this change is most often observed when the
position of the correcting lens is moved from the spectacle plane to the
corneal plane
-Focal Length - -The distance from the vertex of an optical surface to its
focal point The reciprocal of the focal length is the power
, -Focal Point - -The point along the principal axis of an optical surface where
incident parallel light will be focused after being converged or diverged by
the surface
-Focus - -The point along the principal axis of an optical surface that light
converges to or diverges away from after encountering that surface
-Image - -An optical reproduction of an object by a lens or mirror. REAL
images are formed where converging light focuses, and VIRTUAL images are
formed where diverging light focuses
-Index of Refraction - -The ratio of the speed of light in a vacuum (or air) to
the speed in another medium indicating the medium's refractive ability
-Infinity (optical) - -The distance light waves must travel from a source to
lose all curvature and be essentially parallel This distance is considered to be
twenty feet
-K Readings - -Also K's The measurement of the curvature of the central
cornea as measured by the Keratometer
-Keratoconus - -A cone-shaped deformity of the cornea
-Lacrimal Lens - -The lens that is formed by the tears that collect between
the base curve of a contact lens and the anterior cornea
-Macula - -The most sensitive part of the retina Theoretically the macula
would be located where the principal axis intersects the retina
-Normal - -A line drawn perpendicular to the surface
-Object - -The source of light being imaged by an optical system
-Pencil - -A small cluster of light rays
-Power - -The ability of an optical surface to change the vergence of light
incident upon it (See Vertex Power)
-Principal Meridians - -The meridians of MOST and LEAST curvature or
GREATEST and WEAKEST power on a cylindrical or toric surface
-Prism - -An optical device with non-parallel sides The thinnest end is called
the APEX and the thickest end is the BASE
-Prism Ballast - -A prismatic shape used in a contact lens to stabilize the
lens by preventing rotation
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