Optics (Visual-System 2) Flashcards
What is refraction?
The change in the direction of a wave passing from one medium to another caused by its change in velocity.
The frequency of the wave remains the same.
V = f x wavelength
Given frequency is constant, , V is directly proportional to wavelength.
What is the refractive index?
N= speed of light in a vacuum/ speed of light in a medium
c/v
What is the refractive index of air?
1
What happens to light at the boundary at two media?
There is partial reflection and refraction (Depending on the angle of incidence and the critical angle of the medium).
Incidence > critical angle, there is refraction
Incidence = Critical angle, the angle of refraction is 90 degrees.
What are the two types of lenses?
Convex and concave
What does a converging lens do?
A converging lens takes light rays and brings them to a focal point.
What is a diverging lens?
Takes light rays and spreads them outward.
What is emmetropia?
Parallel light rays fall on the retina (there is no accommodation) .
Adequate correlation between axial length and refractive power.
-Does not require correction
What is ametropia?
Refractive error
Mismatch between axial length and refractive power.
The parallel light rays don’t fall on the retina (no accommodation).
What is myopia?
Near-sightedness
Converge at focal point anterior to the retina.
Why is hyperopia?
Farsightedness
What are the common causes of myopia?
Excessive long globe (axial myopia)
Excessive refractive power (refractive myopia)
What are the common symptoms of myopia?
Blurred distance vision
Squint in an attempt to improve uncorrected visual acuity when gazing into the distance
Headache
How can myopia be treated?
1) Correcting with diverging lens (negative lens)
2) Correction with contact lens
3) Correction by removing the lens to reduce refractive power of the eye.
What is hyperopia?
Parallel rays converge at a focal point posterior to the retina
What is the main cause of hyperopia?
Excessive short globe (axial hyperopia)
Insufficient refractive power (refractive hyperopia)
What are the main symptoms of hyperopia?
Visual acuity at near tends to blur relatively early.
Nature of blue is vary from inability to read fine print to near vision is clear by suddenly and intermittently blur.
Blurred vision is more noticeable if person is tired, printing is weak or light inadequate.
Asthenopic symptoms: eyepain, headache in frontal regions, burning sensation in eyes, blepharoconjunctivitis
At what distance does visual acuity diminish in patients with hyperopia?
Near distanced
Where is the headache pain located with hyperopia asthenopic symptoms?
Frontal region
What is abmlyopia?
Uncorrected hyperopia > 5D.
What is hyperopia?
Farsightedness (hyperopia) is a common vision condition in which you can see distant objects clearly, but objects nearby may be blurry.
Which lens should be prescribed in patients with hyperopia?
Correction with converging (Positive lens)
Correction with positive lens + cataract extraction
Correction with contact lens
Correction with intraocular lens
What is astigmatism?
A defect in the eye or in a lens caused by a deviation from spherical curvature, which results in distorted images, as light rays are prevented from meeting at a common focus.
Parallel rays come to focus in 2 focal lines rather than a single focal point.
What is the cause if astigmatism?
Refractive media is not spherical, refract differently along one meridian than along meridian perpendicular to it– Focal points (Punctiform object is is represent as 2 sharply defined lines)
What is the circle of confusion
The two meridians (I and II) are perpendicular to each other. A punctiform object (o) is represented as a line segment Y0 And Y1 at the focal points of the two meridians. Midway between these two focal points is the “circle of least confusion” (Kr), the location with the least loss of image definition
what are the four main symptoms of astigmatism?
1) Asthenopic symptoms (headache and eyepain)
2) Blurred vision
3) Distortion of vision
4) Head tilting and turning
What is the treatment of regular astigmatism?
Cylinder lenses with or without spherical lenses (convex or concave), Sx
What is the treatment of irregular astigmatism?
Rigid cylinder lenses, surgery
What happens to the pupil in the near response triad?
Pupillary miosis, the pupil constricts increasing the depth of field of the optics of the eye
Which muscles mediate the pupil constriction?
The circular sphincter pupillae muscles within the iris
Which muscles assist with convergence?
Medial recti
What is convergence in the near response triad?
Aligns both eyes towards a new object.
Both eyes adduct medially at the same time
What is accommodation in the near response triad?
Increases the refractive power of the lens for near vision
What is the resultant effect with increasing the depth of field?
The eye optical system is able to maintain a clear focus over a certain range of viewing distance even without relying on accommodation
What is the effect on vision in eyes with shallow depth?
Focus is easily lost even with the slightest object movement
What is presbyopia?
The natural occurring loss of accommodation (focus for near objects)
The lens looses its elasticity
What is the average age of onset for presbyopia?
40 years
Which type of vision is intact in presbyopia?
Distant vision
How can presbyopia be treated?
Reading glasses with extra-refractive power for near vision
Convex glasses in near vision:
1) Bifocal
2) Trifocal
3) Progressive power glasses
4) Reading glasses
Which type of lenses can be used to correct presbyopia?
Monofocal lenses, spherical lenses, cylindrical and multi-focal lenses.
What are the advantages to using contact lenses?
higher quality of optical image and less influence on the size of retinal image than spectacle lenses
indication : cosmetic , athletic activities , occupational , irregular corneal astigmatism , high anisometropia , corneal disease
What are the disadvantages of using contact lenses?
disadvantages : careful daily cleaning and disinfection , expense
What are the possible complications with using contact lenses?
complication : infectious keratitis , giant papillary conjunctivitis , corneal vascularization , severe chronic conjunctivitis
What are the possible surgical corrections for presbyopia?
Keratorefractive surgery :RK, AK, PRK, LASIK, ICR, thermokeratoplasty
Intraocular surgery : clear lens extraction (with or without IOL), phakic IOL
What is the first stage of surgical correction in presbyopia?
Initial cutting of the corneal flap using a microblade
What is the purpose of photorefractive treatment (laser) in surgical correction of the lens?
The corneal stroma is reshaped post laser
What are the 8 stages of surgical correction of the lens?
Pre operative eye Initial cutting of corneal flap Cutting of corneal flap Flipping of corneal flap Photorefractive treatment (laser) Corneal stroma reshaped post laser Corneal flap back in position Treatment completed
What surgical correction can be done to correct patients with myopia and astigmatism?
Staar intra-collamer lens (ICL) is inserted into the eye
What is clear lens extraction and IOL?
IOL: Intra ocular lens.
Same as cataract extraction.
Implantation of artificial lens.
Lose accommodation (patient will need reading glasses).
1) The natural lens is removed using a phaco tip
2) An artificial intraocular lens is inserted in the position where the natural lens would sit
