4 - Grover - Visual Optics Flashcards
Refraction
Refractive Index
Bending of light caused by change of velocity of light as it crosses interface between two substances
Refractive Index = Speed in Air / Speed in Substance
Convex Lens
Convex Lens converge parallel light rays
At center of lens, light not bend, angle of incidence increases moving out, leads to greater refracting
- - -
Bends light to single point behind lens
Concave Lens
Concave lens diverge parallel light rays
Center of lens not bent, increasing angle of incident towards edge, greater refraction
- - -
Concave lens bens light away from a single point
Cylindrical Lens
Concave or Convex in only one axis
Light refracted towards or away from a line, not a point
Cylindrical Lenses combined at right angles?
Have same focuses effect as one spherical lens
Focal Length
How does it change with distance of source?
Convexity impact on Focal Length?
Distance behind convex lens at which light rays converge
- - -
The closer th source, the greater the focal length
Refractive Power: Diopter
Diopter = 1 m / Focal Length (m)
+1 Diopter = 1 m Focal Lenth
+2 Diopter = 0.5 m Focal Length
- 1 Diopter diverges light equal to which Concave focuses (focal point on same side as source)
How is refractive power “neutralized”
Combinding Concave and Convex lenses with equal focal strengths (diopters)
Ex. +1 Concave and -1 Convex
Refractive Power - Cylindrical Lenses
Focal Line - Horizontal = Axis of 0 degrees
Focal Line - Vertical = Axis of 90 degrees
Refractive Interfaces of Eye
Air - Surface of Cornea
Back of Cornea - Aqueous Humor
Aqueous Humor - Surface of Lens
Back of Lens - Vitreous Humor
Greatest Refractive Power in Eye?
2/3 Air and Cornea
1/3 Lens
Accommodation
Change in refractive power by changing curvature of lens
Range of Accommodation in Lens
20 d - 34 d
Only in young children, range decreases with age
Nature of Lens in Resting Conditions
Ligaments Tense = Lens Flat = Less Power
Nature of Lens in Contracted Conditions
When contracted ligaments are pulled anterior/medially, releasing tension, more convex = more power
Circular fibers function like a sphincter
Contracted = Relaxed Ligaments = More Convex = More Power
How is lens accomodation controlled?
When does it activate?
Is it volunary or reflexive?
Parasympathetic Control
Ciliary Muscles regulated by parasympathetic nervous system
Parasympathetic activity causes ciliar muscles to contract = ligaments relaxed = more convex = increased power
- - -
Adapts (fires) for viewing nearby objects
Decreases for viewing far away objects
- - -
Voluntary AND Reflexive
Accomodation Change with Age?
Ability to accommodate decreases
Age - Range
0-10: 14 D
50: 2 D
70: 0 D
Iris
Control?
Controls amount of light entering the eye
Autonomic Nervous system has reflex control
Can also affect depth of focus
Sympathetic Activation - Pupil Dilation (mydriasis)
Parasympathetic Activation - Pupil Constriction (miosis)
Depth of Focus (Depth of Field)
Range of distance over which objects remain in focus
Objects nearer or further from point of fixation are in varying degrees of focus
Pupil Diamete effect on Depth of Field
Small Pupil = Long Depth of Focus (little scattering outside plane of focus)
Large Pupil = Short Depth of Focus (scattering outside plane of focus)
Near Triad (Near Reflex)
When eyes fixate on nearby object:
C.A.M.
Convergence: Center object on fovea
Accomodation: Focus object
Miosis: (pupil contriction) to maximize depth of focus
Argyll Robertson Pupil
Bilateral small pupils that constrict when patient focuses on near object, but do not contrict when exposed to bright light
Strongly associated with neurosyphyllis
Clinical: Emmetropia
Normal (emmetropic) eye
Parallel light rays are in sharp focus on retina when ciliary muscle is completely relaxed
Hyperopia vs Myopia
Hyperopia (Farsightedness) - Short eyeball, or weak lens, degraded vision for near objects
Focal point beyond retina
Myopia (Nearsightedness) - Long eyeball, or strong lens, degraded vision for far objects
Focal point in front of retina
***Just remember you got that Myopia, long eyeball, and strong lens
Correcting Errors of Refraction
Hyperopia (can see far) - Correct with ADDED refractive power
+ D (Concave)
- - -
Myopia (can see near) - Correct with REDUCED refractive power
- D (Convex)
- - -
Just remember your prescription.
Astigmatism
Light from one plane is focused at a different distance that light from a secon plane due to uneven curvature of cornea
- - -
Must be corrected with spherical and cylindrical lenses
Intraocular Pressure
Created by fluid within eye
Aqueous Humor: In front and sides of lens (free flowing)
Vitreous Humor: Between lens and retina (gelatinous)
Exachange via diffusion
Formation of Aqueous Humor
Continuously formed and reabsorbed
Secreted by Ciliary Body (ciliary processes)
- - -
Balance between formation and reabsorption determines intraocular pressure
Formation of Aqueous Humor - Process?
Formed on surface of Ciliary Processes by Secretory Epithelial Cells
Similar mechanisms for secretion of aqueous humor and CSF
Path of Aqueous Humor?
- Between suspensory ligaments
- Through pupil into anterior chamber
- Into angle between cornea and iris
- Through trabecular meshwork
- Out of eye through canal of Schlemm
- Into venous circulation
Glaucoma
Damage to the optic nerve (retinal ganglion cells)
Associated with intraocular pressure
Typical Cause: Decreased Outflow of aqueous fluid, elevated pressure may compress optic nerve
Two Types of Glaucoma
- Angle-Closure (narrow angle) Glaucoma:
Iris blocks flow of intraocular fluid, rapid rise in pressure, rapid damage to optic nerve
- Open Angle Glaucoma:
Flow of intraocular fluid impeded at trabecular meshwork, more common, slower rise
