Chapter 5: Optics and Refractive States of the Eye Flashcards
Optics
The branch of physical science that deals with the properties of light and vision
Physical Optics
The study of optics that describes the nature of light in terms of its wave properties
Geometric Optics
The area of optics that deals with the transmission of light as rays and is concerned with the effect of lenses on light and the production of images
Electromagnetic Radiation
Radiation produced through the combination of electrical and magnetic forces, includes rays from the shortest to the longest wavelengths, both visible and invisible
Opaque
Refers to a substance that completely blocks light from passing through it
Translucent
Refers to a substance that transmits light, but significantly interferes with its passage
Transparent
Refers to a substance that transmits light without disrupting its passage
Refracted
Refers to the change in directions of a light when it passes from one medium to another
Optical Density
A lens property that increases the effectiveness to bend light
Refractive Index
The ratio of the speed of light in a vacuum to its speed through a specific substance
Formula= Speed of light in vacuum divided by the speed of light in a specific substance
Prism
A triangular piece of glass or plastic with flat sides, an apex, and a base.
When light passes through a prism, it is bent in a different direction (towards the base)
Refractive power depends on refractive index and size of its apex angle. 1 prism D deviates light 1cm
Plane
Flat
Apex
The top of the prism; pointed
Base
The bottom of the prism; wider
Converge
To come together
Diverge
To spread apart
Convex Lens
A piece of glass or plastic in which one or both surfaces are curved outward
Also called a Positive, or Plus Lens
Concave Lens
A piece of glass or plastic in which one or both surfaces curve inward
Also called a Negative Lens, or Minus Lens
Parallel
Refers to rays that travel side by side in the same direction, neither diverging nor converging
Principle Axis
The pathway of a light ray that strikes the center of a lens of any shape and passes undeviated through the lens material
Principle Rays, Or Axial Rays
A light ray that strikes the center of a lens of any shape and passes undeviated through the lens material
Paraxial Rays
Light rays that enter a lens system away from the center
In Convex lenses, Paraxial rays are brought inward to converge at some point along the Principle Axis
In Concave lenses, Paraxial Rays diverge away from the Principle Axis, and instead produce a virtual image
Focal Point
The point somewhere along the principle axis at which the paraxial rays from a distant source are refracted by a lens and converge in the case of a convex lens and diverge in the case of a concave lens
Plus Lenses
Also called a Convex Lens, or Positive Lens
Minus Lenses
Also called Concave Lenses or Negative Lenses
Virtual Image
The image formed by a concave lens when the paraxial rays from a distant source are refracted and diverge
Vergence Power
Also called Power, it is the measure of a lens’ ability to converge or diverge light rays
Focal Length
The distance between the focal point and the lens
Diopters
The unit of measurement of Power of a lens
D=1/f (focal length)
Accommodation
The change in the curvature of a crystalline lens that helps to focus images of objects close to the eye
Refractive State
The relative ability of the refractive components of the eye to bring objects into focus on the retina
Emmetropia
The refractive state of the eye that is able to focus correctly without the need for corrective lenses
Ametropia
The refractive state of the eye that is unable to focus correctly due to a refractive error and requires corrective lenses
Refractive Error
A nonpathological deficiency in the eye’s optical system
Presbyopia
The progressive loss of Accommodation, due to again
Myopia
Nearsightedness=able to see close, but not far away
Light rays are focused in front of the retina (too much plus power)
Requires Concave lenses to correct
Hyperopia
Farsightedness= able to see far away, but not close
Light rays are focused behind the retina (too much minus power)
Requires Convex lenses to correct
Astigmatism
The refractive error of the eye whose corneal surface curvature is greater in one meridian than another
Both distant and near objects will appear distorted
Spherical Cornea
A cornea whose curvature in all meridians is uniform
Toric Cornea
A cornea whose surface curvature is not uniform
Light rays are not focused into a single point
Principal Meridians
The meridians of maximum and minimum corneal curvature, these meridians are at 90 degrees from each other
Regular Astigmatism
The most common form of astigmatism, in which the cornea resembles a football standing on one end of on its side
Irregular Astigmatism
The less common form of astigmatism, in which the corneal surface losses its uniformity secondary to scarring or other pathology
Types of Corrective Lenses
Glasses, contact lenses, refractive surgery, or surgical implantation of a corrective lens
Spherical Lens
A concave or convex lens whose curvature is uniform
Bifocals
A lens that has 2 powers, typically one for correcting distance vision and one for correcting near vision
Trifocals
A lens that has 3 powers: one for correcting distance vision, one for correcting intermediate vision, and one for correcting near vision
Multifocal Lenes
Any lens that has more than one power
Add
The portion of the multifocal lens that provides correction for near vision
Cylindrical Lenses
A lens that has curvature in only one meridian, commonly used to help correct Astigmatism
Axis
The meridian perpendicular to the meridian with the curvature in cylindrical lens
Spherocylinder Lens
A combination of spherical and cylindrical lenses
Also called a Toric Lens
Does not refract light rays into a single point, but into 2. The clearest point is called the Circle of Least Confusion
Toric Lens
Also called a Spherocylinder Lens
Lensometry
The measurement of certain qualities of lenses by use of a lensometer
Refraction
The process of measuring a patient’s refractive error. Combination of Refractometry and Clinical Judgement
Steps: Retinoscopy, Refinement, Binocular balancing
Retinoscopy
The use of a Retinoscope to determine a refractive error
1st step in Reffraction
Refinement
The subjective 2nd step of Refraction, requiring patient participation and responses, which confirms the information produced with Retinoscopy
Also called Subjective Refractometry or Manifest Refractometry
Binocular balancing
A procedure performed on both eyes at once to ensure that the optical correction determined by refractometry for distance vision does not include an uneven overcorrection or undercorrection
Cycloplegic Refraction
Refractometry performed with the use of a drug that temporarily paralyzes the ciliary muscle, thus blocking accommodation
Done in children frequently
Manifest Refraction
Refractometry performed without the use of cycloplegic drugs
Objective Refractometry
Also called Retinoscopy
Retinoscope
A handheld instrument for measuring refractive error, consists of a light source and a viewing component, and is used in conjunction with a phoropter or trial lenses
Fixate
To gaze steadily at something
Plano Mirror Effect
The flat lighting effect of a retinoscope that produces slightly divergent rays
Concave Mirror Effect
The lighting effect of a retinoscope that produces convergent rays
With Motion
The retinoscopic reflex movement that is in the same direction as the movement of the streak of light from a retinoscope
Typically seen in a Hyperopic eye
Against Motion
The retinoscopic reflex movement that is in the opposite direction from the movement of the streak of light from the retinoscope
Typically seen in Myopic eyes
Neutralization Point
The lens power that is the approximate correction for a refractive error. This is the goal of Retinoscopy
Trial Lens Set
A set of various hand-held lenses introduced before a patient’s eye to select the appropriate corrective lenses
These lenses can be set into a trial frame set
Phoropter, or Refractor
An instrument for determining a corrective lens prescription; stores a range of trial lenses that can be dialed into position
Subjective Refractometry
Also called Refinement
Cross Cylinder
A special lens consisting of 2 cylinders of equal power, one minus and one plus, with their axes set at right angles to each other; used for determining the axis and power of an astigmatic correction
OD
Right eye (oculus dexter)
OS
Left eye (oculus sinister)
OU
Both eyes (oculus uterque)
Transposition
The conversion of a lens prescription from a plus-cylinder to a minus-cylinder
Steps:
1. Algebraically add cylinder power to sphere power
2. Reverse sign of cylinder power
3. Add/Subtract 90 degrees from axis
Lensmeter
An instrument for measuring the prescription of glasses
Also called Lensometer, Vertometer, or Focimeter
Steps:
1. Focus eyepiece
2. Position lens on the stage
3. Measure sphere power (3 thin lines)
4. Measure cylinder power (3 thick lines)(Turn dial towards you for plus cylinder or away from you for minus cylinder)
5. Note axis
6. Measure multifocal power by repositioning glasses (thick to thick lines, or thin to thin lines)
7. Measure Prism (if present)
Vertometer
A device for measuring the distance between the posterior surface of the glass lens and anterior surface of the eyeball
Also called Distometer
Keratometry
The measurement of corneal curvature
Keratometer, or Ophthalmometer
An instrument used to measure corneal curvature
