Refraction and Refractive defects Flashcards
index of refraction formula
n=(speed of light in a vacuum (c))/(speed of light in substance (v))
in which direction does light bend when slowing down between mediums?
bends towards the normal
some light may reflex and refract upon entering a new medium
difference between a convex and concave lens?
convex lens focus light
concave lens spread light
what is the normal condition of parallel rays falling on the retina?
emmetropia
there is adequate correlation between axial length and refractive power
no accommodation is required
what is the opposite of emmetropia (normal)?
ametropia where their is a mismatch in axial length and refractive power
what are the 3 forms of ametropia?
1) myopia (near-sightedness) or hyperopia (far-sightedness)
2) astigmatism
3) presbyopia
what causes near sightedness?
in myopia, the rays converge at a point anterior to the retina (fall short)
unclear aetiology, maybe genetic
causes of myopia (ametropia)
axial myopia
llong eyeball/globe is more common
refractive myopia (excessive refractive power)
treatments for myopia
concave lens (disperse light)
contact lens
laser treatment to remove part of the lens
where do the rays of light land in hyperopia?
rays converge posterior to the retina (maybe genetic)
causes of far sightedness?
- axial hyperopia (short eyeball) is more common
- refractive hyperopia (insufficient refractive power)
what is the result of incorrect hyperopia?
amblyopia (> 5 dioptres)
symptoms of hyperopia
visual acuity blurs early
asthenopic symptoms: eye pain, frontal headache, blepharoconjunctivitis
treatment of hyperopia?
convex lens (converge rays) to increase refractive power
remove lens with laser treatment
have greater powerless contact lens or intraocular lens
what occurs in astigmatism?
oval shaped cornea so rays focus on 2+ focal lines rather than 1 (multiple planes)
the refractive index differs with each plane
hereditary
cause of astigmatism
problem with refractive media i.e. lens not spherical so rays refract differently on their planes (horizontal and vertical) therefore produce 2 focal points
symptoms of astigmatism
asthenopic symptoms
blurred, distorted vision
head tilting and turning
treatment of astigmatism (regular and irregular forms)
specially made lens to adjust for the multiple plane created by the oval shaped cornea
cyclindrical lenses (with or without spherical ones) for regular astigmatism rigid contact lenses (With or without surgery) for irregular astigmatism
what mediates the response for near vision?
what are the components of the near vision response triad? what muscles are involved?
near-response triad via the oculomotor nerve’s PNS branches coming from the inferior branch
- miosis (constriction)
- ->sphincter pupillae= miosis to increase depth of field
- accomdation
- ->circular ciliary muscle= accommodation to increase refractive power of the lens
- convergence
- ->medial rectus= align both eyes towards a near object
what is the naturally occurring loss of accommodation with age?
presbyopia
>40 years, distant vision remains intact
correction and treatment of presbyopia
reading glasses (convex) to increase refractive power bifocal, trifocal and progressive power glasses
what are the 2 types of spectacle lenses?
o Monofocal lenses – spherical or cylindrical lenses.
o Multifocal lenses.
what are the uses of contact lens?
higher quality optical image
without impacting retinal image like with specs
used in sports
aesthetic reasons
complications of contact lenses
infectious keratitis
giant pupillary conjunctivitis
severe chronic conjunctivitis
what are the uses of intraocular lenses?
- replace cataracts crystalline lens
- gives best optical correction for aphakia
- avoid magnification/disotortion caused by specs
two example of surgical procedures to correct refractive defects
1) keratorefractive surgery
2) intraocular surgery
what is involved in intraocular surgery?
clear lens extraction with or without intra-ocular lens.
A small flap of the eye is made and then the laser changes the refractive power of the cornea and the epithelial flap is then re-sewn back onto the eye.
An extra intra-ocular lens can be placed on top of the physiological lens of the eye or the old lens can be removed by laser activity (phaco tip).
what muscle is involved in accomodation?
circular ciliary muscle (contraction)
mechanism of accommodation
contraction of ciliary relaxes the lens zonules (usually stay taught)
mediated by CN III
when there is no tension, the lens returns to normal convex shape (bulges) to increase natural power
how does the lens thickness change for near vision?
becomes thicker as ciliary muscles contract
