Refraction and Refractive defects

Question 1

index of refraction formula

Answer 1

n=(speed of light in a vacuum (c))/(speed of light in substance (v))

Question 2

in which direction does light bend when slowing down between mediums?

Answer 2

bends towards the normal

some light may reflex and refract upon entering a new medium

Question 3

difference between a convex and concave lens?

Answer 3

convex lens focus light

concave lens spread light

Question 4

what is the normal condition of parallel rays falling on the retina?

Answer 4

emmetropia
there is adequate correlation between axial length and refractive power
no accommodation is required

Question 5

what is the opposite of emmetropia (normal)?

Answer 5

ametropia where their is a mismatch in axial length and refractive power

Question 6

what are the 3 forms of ametropia?

Answer 6

1) myopia (near-sightedness) or hyperopia (far-sightedness)
2) astigmatism
3) presbyopia

Question 7

what causes near sightedness?

Answer 7

in myopia, the rays converge at a point anterior to the retina (fall short)

unclear aetiology, maybe genetic

Question 8

causes of myopia (ametropia)

Answer 8

axial myopia
llong eyeball/globe is more common

refractive myopia (excessive refractive power)

Question 9

treatments for myopia

Answer 9

concave lens (disperse light)
contact lens
laser treatment to remove part of the lens

Question 10

where do the rays of light land in hyperopia?

Answer 10

rays converge posterior to the retina (maybe genetic)

Question 11

causes of far sightedness?

Answer 11

• axial hyperopia (short eyeball) is more common

- refractive hyperopia (insufficient refractive power)

Question 12

what is the result of incorrect hyperopia?

Answer 12

amblyopia (> 5 dioptres)

Question 13

symptoms of hyperopia

Answer 13

visual acuity blurs early

asthenopic symptoms: eye pain, frontal headache, blepharoconjunctivitis

Question 14

treatment of hyperopia?

Answer 14

convex lens (converge rays) to increase refractive power

remove lens with laser treatment
have greater powerless contact lens or intraocular lens

Question 15

what occurs in astigmatism?

Answer 15

oval shaped cornea so rays focus on 2+ focal lines rather than 1 (multiple planes)
the refractive index differs with each plane

hereditary

Question 16

cause of astigmatism

Answer 16

problem with refractive media i.e. lens not spherical so rays refract differently on their planes (horizontal and vertical) therefore produce 2 focal points

Question 17

symptoms of astigmatism

Answer 17

asthenopic symptoms
blurred, distorted vision
head tilting and turning

Question 18

treatment of astigmatism (regular and irregular forms)

Answer 18

specially made lens to adjust for the multiple plane created by the oval shaped cornea

cyclindrical lenses (with or without spherical ones) for regular astigmatism 
rigid contact lenses (With or without surgery) for irregular astigmatism

Question 19

what mediates the response for near vision?

what are the components of the near vision response triad? what muscles are involved?

Answer 19

near-response triad via the oculomotor nerve’s PNS branches coming from the inferior branch

• miosis (constriction)
• ->sphincter pupillae= miosis to increase depth of field
• accomdation
• ->circular ciliary muscle= accommodation to increase refractive power of the lens
• convergence
• ->medial rectus= align both eyes towards a near object

Question 20

what is the naturally occurring loss of accommodation with age?

Answer 20

presbyopia

>40 years, distant vision remains intact

Question 21

correction and treatment of presbyopia

Answer 21

reading glasses (convex) to increase refractive power 
bifocal, trifocal and progressive power glasses

Question 22

what are the 2 types of spectacle lenses?

Answer 22

o Monofocal lenses – spherical or cylindrical lenses.

o Multifocal lenses.

Question 23

what are the uses of contact lens?

Answer 23

higher quality optical image
without impacting retinal image like with specs

used in sports
aesthetic reasons

Question 24

complications of contact lenses

Answer 24

infectious keratitis
giant pupillary conjunctivitis
severe chronic conjunctivitis

Question 25

what are the uses of intraocular lenses?

Answer 25

• replace cataracts crystalline lens
• gives best optical correction for aphakia
• avoid magnification/disotortion caused by specs

Question 26

two example of surgical procedures to correct refractive defects

Answer 26

1) keratorefractive surgery

2) intraocular surgery

Question 27

what is involved in intraocular surgery?

Answer 27

clear lens extraction with or without intra-ocular lens.
 A small flap of the eye is made and then the laser changes the refractive power of the cornea and the epithelial flap is then re-sewn back onto the eye.
 An extra intra-ocular lens can be placed on top of the physiological lens of the eye or the old lens can be removed by laser activity (phaco tip).

Question 28

what muscle is involved in accomodation?

Answer 28

circular ciliary muscle (contraction)

Question 29

mechanism of accommodation

Answer 29

contraction of ciliary relaxes the lens zonules (usually stay taught)

mediated by CN III

when there is no tension, the lens returns to normal convex shape (bulges) to increase natural power

Question 30

how does the lens thickness change for near vision?

Answer 30

becomes thicker as ciliary muscles contract