Visual System Optics Flashcards
What is refraction?
light passing through one medium to another, the velocity changes
How can the ratio of the speed changes in refraction be written?
n = Speed of light in a vacuum / Speed of light in a medium
The denominator will ALWAYS be SMALLER and produce a unitless value greater or equal to 1
What is a mediums index of refraction??
The ratio of the speed of light changing from the vacuum to the medium
What happens when light reaches a new medium?
Some light reflects some light refracts
Creates an angle of Incidence = angle of reflection
What two types of lenses are there?
Convex : converging lens takes light and brings them to a point. positive lens
Concave : diverging leans take lights rays and spreads them outward, negative lens
What is Emmetropia?
Adequate correlation between axial length and refractive power
Parallel light fall on the retina with no accommodation
as it should be
What is Ametropia?
Refractive error
Mismatch between axial length and refractive power
SO parallel rays do not fall on retina - no accommodation
Give examples of Ametropia?
- near sightedness = myopia
- Farsightedness = Hyperopia
- Astigmatism
- Presbyopia
What is myopia?
parallel rays converge at the focal point anterior to the retina
caused by:
- excessive long globe
- excessive refractive myopia ( more common )
What are the symptoms of myopia?
Blurred distance vision
Squint to improve visual acuity when gazing into distance
Headache
What three methods can be used to correct myopia?
Correction with diverging lens
Correction with contact lens
Correction by removing the lens to reduce refractive power of the eye
What is hyperopia?
Parallel rays converge at focal point posterior to retina
Caused by:
- excessive short globe (axial hyperopia, more common)
- Insufficient refractive power (refractive hyperopia)
What are the symptoms of hyperopia?
Visual acuity at near tends to blur relatively early
- blur could be inability to read to sometime blurring
- more noticeable if person is tired, light inadquate
Asthenopic symptoms : eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis
Amblyopia – uncorrected hyperopia > 5D
How to treat hyperopia?
Correcting with converging lens
or correction with a positive lens. + cataract extraction
Correction with contact lens
Correction with intraocular lens ( can be in anterior or posterior chamber )
What is Astigatism?
Parallel rays come to focus in 2 focal lines rather than in a single point
Hereditary
Due to refractive media not spherical which changes refraction along one meridian than along the meridian perpendicular to it
- 2 focal points (punctiform object is represented as 2 sharply defined lines)
What are the symptoms of astigmatism?
Asthenopic symptoms
Blurred vision
disrotrion of vision
head tilting and turning
How to treat astigmatism?
to treat regular astigmatism : cylinder lenses or convex/concave
Irregular astigmatism : rigid cylinder lenses, surgery
What are asthenopic symptoms?
headache
eyepain
What is the near response triad?
The pathways that mediate near vision
These are three simultanoues pathways
–> pupillary miosis - pupil constricts increasing the depth of field of the optics of the eye
- this is mediated by the contraction of the circular sphincter pupillae muscles in iris
–> Convergence - medial recti both align both eyes towards a near object
–> accommodation - lens increases refractive power to focus on near object through circular ciliary muscle
What if eyes have a shallow depth of field?
Lose focus easily even with slight object movement
when viewing a near object
What is Presbyopia?
Naturally occuring loss of accommodation
onset from 40+
corrected by reading glasses
convex
to increase refractive power of eye
Why is the age of onset for presbyopia 40?
As we age natural lens looses its elasticity and so ability to accommodate is lost
Vision is not as clear when at near
What types of convex glasses can be given for presbyopia?
reading
bifocal
trifocal
progressive power glasses
What spectacle glasses can be given for presbyopia correction?
Monofocal
Spherical
Cylindrical
Multifocal lenses
Why may contact lenses be given?
higher quality of optical image, less influence on size of retinal image than spectacle image,
can be a cosmetic decision, atheltic activities, occupations, irregular corneal astigmatism, high anisometropia, corneal disease
What are the disadvantegs to contact lenses?
careful daily cleaning and disinfection
expense complication
infectious keratitis
giant papillary conjuctivitis
corneal vascularization
severe chronic conjuctivitis
Why may intraocular lenses be considered?
replacement of catarect crystalline lens
give best optical correction for aphakia, avoid significant magnification and distortion caused by spectacle lenses
What types of surgeries are avaible for correction?
Keratorefractive
Intraocular surgery
Give types of keratorefractive surgeries?
RK AK PRK LASIK IC thermokeratoplasty
Give types of intraocular surgery?
clear lens extraction with or wihtout IOL
Phakic IOL
What are the steps used in surgical correction for LASIK?
- 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 is ICL surgery?
Intra - collamer lens
inserted for the correction of myopia and astigmatism
Why will a px need reading glasses after clear lens extraction + IOL treatment?
same as cataract extraction but also implantation of intra ocular artificial lens
the px loses accommodation so will need glasses
Which of the following is false for myopia?
A) May be associated with large globe
B) Light ray converges behind the retina
C) May be associated with increased corneal curvature
D) Unable to see objects clearly at distance without glasses or other optical correction
= false is B
In accommodation which one of the following does not take place?:
A) Relaxation of Circular Ciliary Muscle
B) Relaxation of Zonules
C) Thickening of Lens
D) Increase of Lens Refractive Power
Relaxation of circular ciliary muscle does not happen
