Visual system optics Flashcards
what is refraction?
change in velocity and path that occurs when light passes through one medium to another
how is the index of refraction calculated?
- Ratio 2 speeds can be compared
- Denominator will ALWAYS be smaller and produce answer > or equal 1 = INDEX OF REFRACTION, n
- all substances have index of refraction which can be used to identify the material
what happens when light reaches a new medium?
some reflects
some refracts through boundary
angle refraction depends on the direction of light
what does the angle of incidence equal?
angle of reflection
what are the 2 types of lenses?
convex lens
concave lens
what does a convex lens do?
converging lens takes light rays and brings them to a point
camera/ eye
what does a concave lens do?
diverging lens takes light rays and spreads them outwards
what is an emmetropia?
perfect eye
adequate correction between axial length and refractive power
parallel light rays fall on the retina (no accommodation)
what is ametropia?
refractive error
mismatch between axial length and refractive power
parallel light rays don’t fall on the retina (no accommodation)
what are the types of ametropia?
- Near sightedness (myopia)
- Farsightedness (hyperopia)
- Astigmatism
- Presbyopia
what is myopia?
- near sightedness
- parallel rays converge at a focal point anterior to retina
- aetology not clear (genetic factor)
- excessive long globe (axial myopia)
- excessive refractive power (refractive myopia)
what are the symptoms of myopia?
- Blurred distance vision
- Squint
- Headache
what is the treatment of myopia?
- Correction with diverging lens (-ve lens)
- Correction with contact lens
- Correction by removing lens to reduce refractive power of eye
what is hyperopia?
long sightedness
- Parallel rays converge focal point posterior to retina
- Aetiology not clear, inherited
- Excessive short globe (axial hyperopia)
- Insufficient refractive power (refractive hyperopia)
what are the symptoms of hyperopia?
- Visual acuity at near tend blur relatively early
- Inability to read fine print but blur can vary from clear to suddenly and intermittently blur
- More noticeable if person is tired, printing is weak or light inadequate
- Eye pain, headache in frontal region, burning sensation in eyes, blepharoconjunctivitis
what is amblyopia?
uncorrected hyperopia >5D
what is the treatment of hyperopia?
- Correction with converging lens (+ve lens)
- Correction with positive lens + cataract extract
- Correction with contact lens
- Correction with intraocular lens
what is astigmatism?
- Parallel rays come to focus in 2 focal lines rather than single focal point
- Aetiology heredity
- Refractive media is not spherical= refract differently along one meridian than along meridian perpendicular to it= 2 points
- Two meridians perpendicular to each other
- Midway between two focal points = circle of least confusion (location with least loss of image definition)
what are the symptoms of astigmatism?
- Asthenopia symptoms (headache, eyepain)
- Blurred vision
- Distortion of vision
- Head tilting and turning
what is the treatment of astigmatism?
- Regular astigmatism: cylinder lens with or without spherical lenses (Convex or concave)
- Irregular astigmatism: rigid cylinder lenses or surgery
what is the near response triad?
adaptation for near vision
what pathways is the near response triad mediated by?
- Pupillary miosis (circular sphincter pupillae contraction mediation within iris), pupil constricts, increasing depth of field of optic of the eye
- Larger field depth= maintain clear focus over certain range of viewing distances without relying on accommodation
- Important for near vision, Eyes with shallow depth loose focus easily
- Larger field depth= maintain clear focus over certain range of viewing distances without relying on accommodation
- Convergence- both eyes adduct medially at same time to align both eyes towards near object
- Mediated by simultaneous contraction of medial rectus extra-ocular muscles from both eyes
- Accommodation- increase refractive power of lens required for near vision focus
- Mediated by circular ciliary muscle
what is presbyopia?
- Naturally occurring loss accommodation – lose elasticity of eye
- Onset from 40years old
- Distant vision intact
what is presbyopia corrected by?
- Corrected by reading glasses (convex lenses) to increase refractive power of eye
what are the types of optical correction?
contact lenses
intraocular lenses
surgical correction
what are the indications of contact lenses?
- Cosmetic, occupational, irregular corneal astigmatism, high anisometropia, corneal disease
what are the disadvantages of contact lenses
careful cleaning and disinfection
expensive
what are intraocular lenses?
- Replacement cataract crystalline lens
- Give best optical correction for aphakia, avoid significant magnification and distortion caused by spectacle lenses
what are the surgical correction options for eye sight?
- Keratorefractive surgery (LASIK)
- intra-collamer lens (correction myopia and astigmatism)
- clear lens extraction & intraocular lens
how does lazer eye surgery work?
where do ICL get implanted?
between lens of eye and iris
what happens in clear lens extraction and intraocular lens replacement?
same as cataract extraction
implantation of artifical lens
what are the negatives of a clear lens extraction?
loose accommodation (require reading glasses)
