Optics of the Visual System Flashcards
Describe the phenomenon of refraction
When light travels through one medium to another it changes velocity causing the ray to refract, this makes it bend
What is the equation for calculating index of refraction?
Speed of light in a vacuum/speed of light in the medium (n= c/Vn)
What should the index of refraction always be?
Greater or equal to 1 because the denominator is always smaller than the numerator
When the light goes from less dense to more dense where does it bend?
Towards the normal
How many types of lens are there? What are they called?
2 types:
Concave
Convex
Describe the properties of a convex lens
Converging lens that causes light rays to come together at a point
Describe the properties of a concave lens
A diverging lens that causes light rays to spread away from each other
What is ametropia?
Refractive error, there is a mismatch between axial length and refractive power
This means parallel rays don’t fall on the retina and there’s no accomodation
What is the medical name for near sightedness?
Myopia
What is the medical name for far sightedness?
Hyperopia
What are the types of ametropia?
Myopia (nearsightedness)
Hyperopia (farsightedness)
Astigmatism
Presbyopia
What happens to light rays in those with myopia?
Light rays converge at a point anterior to the retina
What are the types of myopia? Describe them
Axial myopia= excessively long globe
Refractive myopia= excessive refractive power
What type of myopia is more common?
Axial myopia
What are symptoms of myopia?
Blurred distance vision
Squinting to help overcome this
Headache as a result
How is myopia corrected?
A) Diverging lens (negative power)
B) Contact lens
C) Removal of lens to reduce refractive power
What happens to light rays in those with hyperopia?
Light rays converge at a focal point posterior to the retina
What are the types of hyperopia? Describe them
Axial hyperopia= excessively short globe
Refractive hyperopia= insufficient refractive power
What type of hyperopia is more common?
Axial
Do those with near sightedness struggle to see near or far objects?
Far
Do those with far sightedness struggle to see near or far objects?
Near
Do those with myopia struggle to see near or far objects?
Far
Do those with hyperopia struggle to see near or far objects?
Near
What are symptoms of hyperopia?
Blurred vision when looking at things up close eg reading,
- worse if there is reduced light, print is weak or if person is tired
Asthenopic symptoms: eye pain, headache in frontal region, burning in eyes, blepharoconjunctivitis
What do we call uncorrected hyperopia? And what refraction do they have?
Amblyopia
Refraction above 5D
How is hyperopia corrected?
A) converging lens (positive power)
B) contact lens
C) correction with intraocular lens
D) correction with positive lens and cataract extraction
What happens to light rays in astigmatism?
Parallel rays focus in 2 focal lines instead of at a singular focal point because the refractive media is not spherical
What are symptoms of astigmatism?
Asthenopic symptoms: Headache, eye pain
blurred vision
distorted vision
head tilting/turning
How is astigmatism treated?
Regular astigmatism: Cylinder lens with or without spherical lens (convex/concave)
Irregular astigmatism: rigid cylinder lens, surgery
What is the near response triad? Describe it
Its how the eye adapts to look at close objects
1) pupillary miosis (by sphincter pupillae) to increase depth of field
2) convergence (by medial recti of both eyes) to align both eyes towards a near object
3) accomodation (by circular ciliary muscle) to increase the refractive power of the lens
What is presbyopia?
Naturally occuring loss of accommodation
Distant vision intact
What is accomodation?
Focus on near objects
At what age is there usually onset of presbyopia?
40 years
How is presbyopia corrected?
Convect lenses: -Reading glasses -Bifocal glasses -Trifocal glasses -Progressive power glasses Contact lenses: Given for cosmetic, athletes, occupational, irregular corneal astigmatism, high anisometropia, corneal disease
What are the 3 main types of optical correction?
Contact lens
Intraocular lens
Surgical correction
What are some advantages of contact lens?
Higher quality of optical image, influences the size of the image less than spectacles
What are some disadvantages of contact lens?
They need careful daily cleaning and disinfection, they’re expensive
What is the best use of intraocular lens?
The replace the cataract crystalline lens
Optical correction of aphakia (no crystalline lens) as they avoid significant magnification and distortion via spectacles
What happens when the lens is cleared surgically?
Patients loose the ability to accomodate and require reading glasses
What happens to the zonules and lens in accomodation?
Zonules relax
Lens thickens and increases in their refractive power
What is emmetropia?
Adequate correlation between axial length and refractive power
Parallel light rays fall on the retina
What are complications of contact lenses?
Infectious keratitis, giant papillary conjunctivitis, corneal vascularisation, severe chronic conjunctivitis
What are types of surgical corrections?
Keratorefractive surgery: RK,AK, PRK, LASIK, ICR, thermokeratoplasty
Intraocular surgery: clear lens extraction (with or without IOL). phakic IOL
What kind of lens is inserted for the correction of myopia or astigmatism?
Intra-collamer lens (ICL)
What is IOL?
Intra ocular lens
Its the implantation of an artificial lens
