Optics And Refractive States Of The Eye Flashcards
Define the following terms : refractive index, focal point, focal length, and diopter.
Refractive index: the ratio of the speed of light in a vacuum to its speed in a specific substance.
Focal point : the point somewhere along the principal axis of a lens at which the paraxial rays converge or diverge.
Focal Length : the distance between the focal point and the lens.
Diopter : the unit of measure of the power of a lens
Name the principal refractive properties of a convex lens and a concave lens.
A convex lens converges light rays; a concave lens diverges light rays.
Distinguish between emmetropia and ametropia.
Emmetropia : the normal refractive state of an eye, in which light rays for distant objects are focused clearly on the retina by the relaxed lens without any accommodative effort.
Ametropia : the abnormal refractive state of the eye, in which light rays from distant objects cannot be focused clearly on the retina due to a refractive error.
Define and describe the physiologic characteristics of myopia, hyperopia and astigmatism.
Myopia : nearsightedness; the cornea and lens have too much plus power for the length of the non accommodating eye, so that the light rays from distant objects are focused in front of the retina.
Hyperopia : farsightedness; the cornea and lens have too little plus power for the length of the non accommodating eye, so that the light rays from distant objects are focused theoretically behind retina.
Astigmatism : blurred vision of both distant and near objects due to a toric cornea or whose surface is irregular, so light rays are brought to a single focal point.
Describe presbyopia
Progressive loss of accommodative ability of the crystalline lens due to a natural process of aging.
Name at least 3 methods of correcting refractive errors of the eye.
Eyeglasses, contact lenses, intraocular lens implants, refractive surgery
Define the state and purpose of an add power in multi focal lenses
Lower segment(s) of a multifocal lens; used to provide near vision to patients with presbyopia.
Define a spherocylinder
Lens that has different curvatures in each of 2 perpendicular meridians (like a football), each of which possesses refractive power.
State the principal refractive characteristic of a prism
Refracts light rays toward its base
State the purpose of incorporating a prism into a corrective lens
To correct diplopia caused by visual misalignments.
The refractive power of a prism is measured in units called _____.
Prism Diopters
A 0.5 /\ prism deviates parallel rays of light a1 cm at a distance of ______ m from the prism.
2
Name the kind of ophthalmic lens used to correct the following refractive errors : myopia, hyperopia and astigmatism
Myopia : concave, or minus, sphere.
Hyperopia: convex, or plus, sphere.
Astigmatism : spherocylinder, or cylinder.
Define refraction as the term is used in eye care
Process of measuring a persons refractive error and determining the optical correction required to provide clear vision.
Name the 2 main components of refraction
Refractometry and clinical judgement
Name the 3 separate steps of refractometry.
Retinoscopy (objective refractometry), refinement (subjective refractometry) , binocular balancing
Distinguish between cycloplegic refraction and manifest refraction
Cycloplegic refraction : uses cycloplegic drops to temporarily paralyze the ciliary muscle and block accommodation.
Manifest Refraction : is obtained without cycloplegia
Define and distinguish between objective refractometry and subjective refractometry.
Objective Refractometry is retinoscopy, which does not require require responses from the patient.
Subjective Refractometry is refinement, which requires the patient to participate by choosing the lens (usually presented by means of a phoropter or trial frame and lenses) that provides the better clarity.
Name 4 instruments or devices that may be used during refraction
Retinoscope, refractor (or phoropter), trial lens set and trial frame, cross cylinder
Define ‘with motion’ and ‘against motion’.
With Motion : the retinoscopic reflex of the eye that moves in the same direction as the retinoscope streak of light.
Against Motion : the retinoscopic reflex of the eye that moves in the opposite direction from the retinoscope streak of light
Name the 3 components of the lens prescription shown
+2.75 -1.50 X135
A. B. C.
A . sign and power of sphere
B. Sign and power of cylinder
C. Axis of Cylinder
Transpose the following lens prescription :
+4.25 +1.50 X 135
+5.75 -1.50 X 45
Define lensometry.
Measurement of the prescription of eyeglasses lenses or the power of contact lenses
List the 4 properties of lenses that can be measured by a lensometer
Power in diopters, axes of cylinder component, presence and direction of prism, optical centers
Name the type of lens identified by lensometry when the single and triple perpendicular line come into focus simultaneously.
Sphere
The lensmeter power-drum reading on the distance portion of a multifocal lens measures -1.50 sph. The power-drum reading for the bifocal is +0.25. What is the bifocal add?
+1.75
Define Keratometry
Measurement of a patients corneal curvature; provides an objective, quantitative measurement of corneal astigmatism
List the 4 conditions that must be met simultaneously to obtain a proper measurement with a keratometer.
- The reticule is in the center of the bottom-right circle.
- The instrument is focused with fusion of the bottom-right circles.
- The pulses between the circles are in the same plane and are fused.
- The minuses between the circles are in the same plane and are fused.
