Visual System - Optics

Question 1

What is refraction?

Answer 1

Occurs when light passes from one medium through to another due to change in velocity

Question 2

What is a refractive index?

What is the formula to work out the refractive index?

Answer 2

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

Question 3

What happens to the light when it hits a new medium?

Answer 3

Some of the light reflects off the boundary

Some of the light travels through the boundary

Question 4

What is the angle of incidence, angle of reflection and angle of refraction?

What is the formula connecting the incidence and reflection?

Answer 4

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

Question 5

What are the 2 types of lenses?

Answer 5

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,

Question 6

What is emmetropia?

Answer 6

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

Question 7

What is ametropia?

Answer 7

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

Question 8

What is myopia, how does it work?

What may cause it?

Answer 8

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

Question 9

How does myopia present clinically?

Answer 9

Blurred distance vision
Squint in an attempt to improve uncorrected visual acuity when gazing into the distance
Headache

Question 10

How is myopia treated?

Answer 10

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

Question 11

What is hyperopia?

Answer 11

Farsightedness
Parallel rays converge at a focal point posterior to the retina

Excessive short globe (axial hyperopia) = more common
Insufficient refractive power (refractive hyperopia)

Question 12

How does hyperopia present clinically?

Answer 12

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

Question 13

What is Amblyopia?

Answer 13

Uncorrected hyperopia of more than 5D in one eye

Question 14

How is hyperopia treated?

Answer 14

Glasses - convex lens brings focal point onto the retina
Contact lenses
Surgery - alter intraocular lens

Question 15

What is astigmatism?

What is the circle of least confusion and where is it located?

Answer 15

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

Question 16

What are the symptoms of headaches?

How can astigmatism be corrected?

Answer 16

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

Question 17

What are the 3 steps that our eyes take to adapt to near vision?

Answer 17

1. Sphincter Pupillae = pupillary miosis (excessive constriction) to increase depth of field
2. Convergence (medial recti from both eyes) to align both eyes towards a near object
3. Circular Ciliary Muscle - accomodation to increase the refractive power of lens for near vision

Question 18

What is Presbyopia?

How can this be corrected?

Answer 18

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

Question 19

What are the 4 main types of optical correction?

Answer 19

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)

Question 20

What are the disadvantages of contact lenses?

What are some complications associated with contact lenses?

Answer 20

Disadvantages = careful daily cleaning and disinfection expense

Complications = infectious keratitis , giant papillary conjunctivitis , corneal vascularization , severe chronic conjunctivitis

Question 21

What are the advantages of intraocular lenses?

Answer 21

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

Question 22

How is the surgical laser treatment carried out?

Answer 22

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

Question 23

What is an intraocular lens implant?

What is the issue with this?

Answer 23

intraocular lens implant placed in front of the natural lens to correct refractive error

Cataract will arrive

Question 24

Is the surgical process for a cataract extraction and clear lens extraction the same?

Answer 24

Yes

Question 25

What is the procedure for a clear lens extraction and intraocular lens insertion?

Answer 25

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