Visual System - Optics Flashcards
What exactly is light doing when it reaches a new medium?
- There is partial reflection and refraction (Depending on the angle of incidence and the critical angle of the medium)
- Incidence > critical angle, there is refraction
- Incidence = Critical angle, the angle of refraction is 90 degrees
What is refraction?
The change in the direction of a wave passing from one medium to another caused by its change in velocity.
The frequency of the wave remains the same.
V = f x wavelength
Given frequency is constant, V is directly proportional to wavelength.
What is a refractive index?
- N = speed of light in a vacuum / speed of light in a medium
- n = c/v
What is the refractive index of air?
- 1
What are the 2 types of lenses?
- Convex and concave
What does a converging lens do?
- A converging lens takes light rays and brings them to a focal point
What is a diverging lens?
- Takes light rays and spreads them outward
What is emmetropia?
- No deficiency:
-
Parallel light rays fall on the retina (there is no accommodation)
- Adequate correlation between axial length and refractive power
- Does not require correction
-
Parallel light rays fall on the retina (there is no accommodation)
What is ametropia?
- Refractive error
-
Mismatch between axial length and refractive power
- The parallel light rays don’t fall on the retina (no accommodation)
-
Mismatch between axial length and refractive power
- Near-sightedness (Myopia)
- Farsightedness (Hyperopia)
- Astigmatism
- Presbyopia
What is myopia?
-
Near-sightedness
- Converge at focal point anterior to the retina
What are the common causes of myopia (2)?
- Excessive long globe (axial myopia)
- Excessive refractive power (refractive myopia)
What are the common symptoms of myopia (3)?
- Blurred distance vision
- Squint in an attempt to improve uncorrected visual acuity when gazing into the distance → Headache
How can myopia be treated (3 options)?
- Correcting with diverging lens (negative lens)
- Correction with contact lens
- Correction by removing the lens to reduce refractive power of the eye
What is hyperopia?
- Farsightedness (hyperopia) is a common vision condition in which you can see distant objects clearly, but objects nearby may be blurry
- Parallel rays converge at a focal point posterior to the retina
What is the main causes of hyperopia (2)?
- Excessive short globe (axial hyperopia)
- Insufficient refractive power (refractive hyperopia)
What are the main symptoms of hyperopia (5)?
- Visual acuity at near tends to blur relatively early.
- Nature of blur is vary from inability to read fine print to near vision is clear by suddenly and intermittently blur
- Blurred vision is more noticeable if person is tired, printing is weak or light inadequate.
- Asthenopic symptoms: eyepain, headache in frontal regions, burning sensation in eyes, blepharoconjunctivitis
How can hyperopia be treated (4 options)?
- Correction with converging lens (Positive lens)
- Correction with positive lens + cataract extraction
- Correction with contact lens
- Correction with intraocular lens
What is astigmatism?
- A defect in the eye or in a lens caused by a deviation from spherical curvature, which results in distorted images, as light rays are prevented from meeting at a common focus.
- Parallel rays come to focus in 2 focal lines rather than a single focal point
What is the cause of astigmatism?
- Refractive media is not spherical, refract differently along one meridian than along meridian perpendicular to it → Focal points (Punctiform object is is represent as 2 sharply defined lines)
Usually heredity
What are the 4 main symptoms of astigmatism?
- Asthenopic symptoms (headache & eyepain)
- Blurred vision of vision
- Head tilting and turning
- Distortion
How can astigmatism be treated (2R / 2IR)?
- Regular astigmatism:
- Cylinder lenses with or without spherical lenses (convex or concave)
- Surgery
- Irregular astigmatism:
- Rigid cylinder lenses
- Surgery
What happens to the pupil in the near response triad?
- Pupillary miosis, the pupil constricts increasing the depth of field of the optics of the eye
Which muscles mediate the pupil constriction?
- The circular sphincter pupillae muscles within the iris
What is convergence in the near response triad?
- Aligns both eyes towards a new object
- Both eyes adduct medially at the same time
Which muscles mediate the convergence?
Medial recti from both eyes
What is accommodation in the near response triad?
- Increases the refractive power of the lens for near vision
Which muscles mediate the accomodation?
Circular Ciliary Muscle
What is presbyopia?
- The natural occurring loss of accommodation (focus for near objects)
- The lens looses its elasticity
What is the average age of onset for presbyopia?
- 40 years
Which type of vision is intact in presbyopia?
- Distant vision
How can presbyopia be treated (2)?
- Reading glasses with extra-refractive power for near vision
-
Convex glasses in near vision:
- Bifocal
- Trifocal
- Progressive power glasses
- Reading glasses
Which type of lenses can be used to correct presbyopia (4)?
-
Monofocal lenses
- Spherical lenses
- Cylindrical lenses
- Multi-focal lenses
What are the advantages to using contact lenses (2)?
- Higher quality of optical image and less influence on the size of retinal image than spectacle lenses
-
Indication:
- Cosmetic
- Athletic activities
- Occupational
- Irregular corneal astigmatism
- High anisometropia
- Corneal disease
What are the disadvantages of using contact lenses (2)?
- Careful daily cleaning and disinfection
- Expense
What are the possible complications with using contact lenses (4)?
- Infectious keratitis
- Giant papillary conjunctivitis
- Corneal vascularization
- Severe chronic conjunctivitis
What are the possible surgical corrections for presbyopia (2)?
- Keratorefractive surgery: RK, AK, PRK, LASIK, ICR, thermokeratoplasty
- Intraocular surgery: clear lens extraction (with or without IOL), phakic IOL
What surgical correction can be done to correct patients with myopia and astigmatism?
- Staar intra-collamer lens (ICL) is inserted into the eye
What are intraocular lenses?
- Replacement of cataract crystalline lens
- Give best optical correction for aphakia, avoid significant magnification and distortion caused by spectacle lenses
What is clear lens extraction and intra ocular lens (IOL)?
- Same as cataract extraction:
- Lose accommodation (patient will need reading glasses)
- The natural lens is removed using a phaco tip
- An artificial intraocular lens is inserted in the position where the natural lens would sit
What are the 8 stages of surgical correction of the lens?
- 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
Which statement is false for Myopia?
1. May be associated with large globe
2. Light ray converges behind the retina
3. May be associated with increased corneal curvature
4. Unable to see objects clearly at distance without glasses or other optical correction
Light ray converges behind the retina
In accommodation, which one of the following events does not take place?
1. Relaxation of Circular Ciliary Muscle
1. Relaxation of Zonules
1. Thickening of Lens
1. Increase of Lens Refractive Power
Relaxation of Circular Ciliary Muscle
