Visual System - Optics Flashcards
What is refraction?
Occurs when light passes from one medium through to another due to change in velocity
What is a refractive index?
What is the formula to work out the refractive index?
How much the light’s velocity changes when it hits a different medium
Speed of light in a vacuum divided by speed of light in a medium
What happens to the light when it hits a new medium?
Some of the light reflects off the boundary
Some of the light travels through the boundary
What is the angle of incidence, angle of reflection and angle of refraction?
What is the formula connecting the incidence and reflection?
Angle of incidence is the angle the light hits the boundary at
Angle of reflection is the angle at which the light reflects at the boundary
Angle of refraction is the angle at which the light travels through the new medium
Angle of incidence = angle of reflection
What are the 2 types of lenses?
Convex - takes light rays and brings them to a focal point at a given distance
Concave - takes light rays and spreads them outwards, has a virtual focal point,
What is emmetropia?
Perfect eye sight as the lens has adequate correlation between axial length and refractive power
Parallel light rays fall on the retina without the need of any accommodation
What is ametropia?
Mismatch between axial length and refractive power
Parallel light rays don't fall on the retina without accomodation, this can cause: Near-sightedness (myopia) Far-sightedness (hyperopia) Astigmatism Presbyopia
What is myopia, how does it work?
What may cause it?
Shortsightedness
Parallel rays converge at a focal point anterior to the retina (image does not fall onto the surface of the retina)
Excessive long globle = most common cause
Excessive refractive power (refractive myopia) - increased corneal curvature
How does myopia present clinically?
Blurred distance vision
Squint in an attempt to improve uncorrected visual acuity when gazing into the distance
Headache
How is myopia treated?
Glasses - concave lens that pushes focus onto the back of the eye
Contact lenses
Surgery - to remove lens to reduce the refractive power of the eye
What is hyperopia?
Farsightedness
Parallel rays converge at a focal point posterior to the retina
Excessive short globe (axial hyperopia) = more common
Insufficient refractive power (refractive hyperopia)
How does hyperopia present clinically?
Visual acuity at near tends to blur relatively early
(blur can be inability to read fine print or intermittent blur)
Blurred vision more noticeable if person is tired, printing is weak or light inadequate
Eyepain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis
What is Amblyopia?
Uncorrected hyperopia of more than 5D in one eye
How is hyperopia treated?
Glasses - convex lens brings focal point onto the retina
Contact lenses
Surgery - alter intraocular lens
What is astigmatism?
What is the circle of least confusion and where is it located?
Parallel rays come to focus in 2 focal lines rather than a single focal point
Because the cornea, a dome shaped lens, has an irregular conformation, therefore multiple focal points may be generated
The point halfway between the 2 focal lengths is called the ‘circle of east confusion’ and this is where the image appears less blurred
What are the symptoms of headaches?
How can astigmatism be corrected?
Asthenopic symptoms (headache , eyepain)
Blurred vision
Distortion of vision
Head tilting and turning
Treatment:
Regular astigmatism = cylinder lenses with or without spherical lenses (convex or concave)
Irregular astigmatism = rigid cylinder lenses, surgery
What are the 3 steps that our eyes take to adapt to near vision?
- Sphincter Pupillae = pupillary miosis (excessive constriction) to increase depth of field
- Convergence (medial recti from both eyes) to align both eyes towards a near object
- Circular Ciliary Muscle - accomodation to increase the refractive power of lens for near vision
What is Presbyopia?
How can this be corrected?
Natural occuring loss of accommodation - after the age of 40
Distant vision remains intact (until cataract kicks in)
Corrected by reading glasses = convex lenses to increase refractive power of the eye
What are the 4 main types of optical correction?
Spectacle lenses (glasses), can be: Monofocal = spherical or cylidrical lenses Multifocal lnses
Contact lenses:
Higher quality of optical image, and less influence on size of retinal image than spectacles (sit on cornea, cannot be moved closer / further away)
May be used for - cosmetic, athletic, occupational, irregular astigmatism etc.
Intraocular lens - Lens implanted in the eye - replacement of cataract crystalline lens
Surgical correction - laser surgery (keratorefractive surgery)
What are the disadvantages of contact lenses?
What are some complications associated with contact lenses?
Disadvantages = careful daily cleaning and disinfection expense
Complications = infectious keratitis , giant papillary conjunctivitis , corneal vascularization , severe chronic conjunctivitis
What are the advantages of intraocular lenses?
Lens implanted in the eye - replacement of cataract crystalline lens
Gives best optical correction for aphakia (absence of crystalline lens)
Avoids significant magnification and distortion caused by spectacle lenses
How is the surgical laser treatment carried out?
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 is an intraocular lens implant?
What is the issue with this?
intraocular lens implant placed in front of the natural lens to correct refractive error
Cataract will arrive
Is the surgical process for a cataract extraction and clear lens extraction the same?
Yes
What is the procedure for a clear lens extraction and intraocular lens insertion?
Ultrasound tip (phaco tip) - breaks natural lens and sucks it up An artificial intraocular lens inserted and place where the natural lens used to sit
