NCLE Glossary Exam Questions With Correct Answers

Accommodation - Answer The ability of the crystalline lens to adjust power to provide retinal focus of images of objects closer than optical infinity Aphakia - Answer Absence of the crystalline lens. Astigmatism - Answer 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) - Answer The weakest or least curved meridian on the surface of a cylindrical or toric surface Axis (principal) - Answer An imaginary line going through the geometric center of an optical system perpendicular to the surface. Base Curve - Answer Of a contact lens the curvature of the posterior optical portion otherwise known as the Posterior Central Curve Cylinder - Answer An object whose surface is curved differently in different meridians with the axis having zero curvature Cylinder Lens - Answer A lens with different power in different meridians due to one or more surfaces having cylindrical or toric shape Cylinder Power - Answer The algebraic difference in power between the principal meridians of a cylinder lens Diopter - Answer A unit of measurement equal to the reciprocal meter used to measure vergence of light and power of optical lenses Dispersion - Answer The separation of white light into its spectral colors Effectivity - Answer 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 - Answer The distance from the vertex of an optical surface to its focal point The reciprocal of the focal length is the power Focal Point - Answer 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 - Answer The point along the principal axis of an optical surface that light converges to or diverges away from after encountering that surface Image - Answer 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 - Answer 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) - Answer 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 - Answer Also K's The measurement of the curvature of the central cornea as measured by the Keratometer Keratoconus - Answer A cone-shaped deformity of the cornea Lacrimal Lens - Answer The lens that is formed by the tears that collect between the base curve of a contact lens and the anterior cornea Macula - Answer The most sensitive part of the retina Theoretically the macula would be located where the principal axis intersects the retina Normal - Answer A line drawn perpendicular to the surface Object - Answer The source of light being imaged by an optical system Pencil - Answer A small cluster of light rays Power - Answer The ability of an optical surface to change the vergence of light incident upon it (See Vertex Power) Principal Meridians - Answer The meridians of MOST and LEAST curvature or GREATEST and WEAKEST power on a cylindrical or toric surface Prism - Answer An optical device with non-parallel sides The thinnest end is called the APEX and the thickest end is the BASE Prism Ballast - Answer A prismatic shape used in a contact lens to stabilize the lens by preventing rotation Prism Diopter - Answer The unit of measurement to describe the power of prism to deviate light One prism diopter produces a deviation of one centimeter at a distance of one meter Radius of Curvature - Answer An expression of the curvature of a surface by stating the length of the curve's radius The shorter the radius the steeper the curve Base curves of contact lenses are usually expressed in this way