vision Flashcards
optics: explain the basis of physiological optics and the common defects of refraction
what is the index of refraction (n)
speed of light in vacuum/speed of light in medium (as denominator always smaller, value is unitless and greater or equal to 1)
what 2 things change when light goes from one medium to another
velocity and direction
what 2 things happen when light reaches a new medium
some reflects off boundary, and some refracts through boundary
what is the angle of incidence equal to
angle of reflection
what is angle of incidence > or < angle of refraction depending on
direction of light
2 types of lenses, shape and what they do
converging (convex) brings light rays to a focal point (distance from lens to focal point is focal length); diverging (concave) spreads light rays from source (focal point) outwards (distance from focal point to lens is focal length)
function of converging lens
focus an image
what is emmetropia
adequate correlation between axial length and reflective power, so parallel light rays fall on retina with no accomodation
what is ametropia
refractive error where there is a mismatch between axial length and refractive power, meaning light rays don’t fall on retina (no accomodation)
4 types of ametropia
nearsightedness (myopia), farsightedness (hyperopia), astigmatism, presbyopia
what happens in myopia
parallel rays converge at focal point anterior to retina
2 causes of myopia
genetic factor: excessive long globe (axial; more common), excessive refractive power (refractive; lens is too convex)
3 symptoms of myopia
blurred distance vision, squint in attempt to improve uncorrected visual acuity when gazing into distance (only get light that is straight so no need for refractive power), headache
3 ways myopia is corrected
diverging lens, by making retina more convex, or by removing lens to reduce refractive power of eye (no longer done as many other issues)
what happens in hyperopia
parallel rays converge at focal point posterior to retina
2 causes of hyperopia
inherited: excessive short globe (axial; more common), insufficient refractive power (refractive)
2 symptoms of hyperopia
visual acuity at near tends to blur relatively early, asthenopic symptoms
when is blurred vision more noticeable
when tired, weak printing, or inadequate lighting
asthenopic symptoms of hyperopia
eyepain, headache in frontal region, burning sensation in eyes, blepharoconjunctivitis
what is ambylopia
uncorrected hyperopia > 5D, so vision is always blurry regardless of distance; lazy eye ir not corrected at childhood as brain doesn’t use that eye as blurred
how is hyperopia corrected
converging lens, anterior chamber intraocular lens, posterior chamber intraocular lens
what happens in astigmatism
parralel rays come to focus in 2 focal lines rather than one, as cornea dome different shape
cause of astigmatism
hereditary; refractive media is not spherical so refracts differently along one meridian than along meridian perpendicular to it, creating 2 focal points on retina (punctiform object represented as 2 sharply defined lines)
4 symptoms of astigmatism
asthenopic symptoms (eyepain, headache), blurred vision, distorted vision, head tilting and turning
treatment of regular astigmatism
cylinder lenses with/without spherical lenses (convex or concave), so light pasing through vertical part will go straight, but light passing through side will hit convex/concave structure, creating joint focal point
treatment of irregular astigmatism
rigid cylinder lenses, surgery
what is the near response triad for
adaption for near vision
3 things responsible for near response triad
pupillary miosis, convergence, accomodation
what causes pupillary miosis and effect
sphincter pupillae to increase depth of field
what causes convergence and effect
medial recti from both eyes to align both eyes towards a near object
what causes accomodation and effect
circular ciliary muscle to increase refractive power of lens for near vision
what is presbyopia
naturally occuring loss of accomodation (focus for near objects) after 40 years, with distant vision intact
how is presbyopia corrected
reading glasses with convex lenses to increase refractive power of eye; also bi/tri focal glasses or progressive power glasses
4 types of optical correction
spectacle lenses, contact lenses, intraocular lenses, surgical
types of spectacle lenses
monofocal: spherical, cylindrical; multifocal
contact lenses vs spectacle lenses
contact lenses have higher quality of optical image and less influence on size of retinal image
indication for contact lenses
cosmetic, athletic activities, occupational, irregular corneal astigmatism, high anisometropia, corneal disease
disadvantages of contact lenses
careful daily cleaning and disinfection, expense
complication of contact lenses
infectious keratitis, giant papillary conjunctivitis, corneal vascularisation, severe chronic conjunctivitis
what is intraocular lenses
replacement of cataract crystalline lens
when are intraocular lenses used
best optical correction for aphakia, and avoid significant magnification and distortion caused by spectacle lenses
2 types of surgical correction
keratorefractive, intraocular
what is clear lens extraction with intraocular lens; what is lost and correction
same as cataract extraction, with implantation of artifical lens; lose accomodation so needs reading glasses
accomodation mechanism for near vision
circular ciliary muscle in ciliary body contracts -> zonules between ciliary body and lens capsule attachment passively relax -> lens returns to natural convex shape due to innate elasticity -> refractive power of lens increases
what mediates accomodation
efferent oculomotor nerve
