Vision 1 - refraction and accommodation

Question 1

what is refraction

Answer 1

bending of light as it passes from one optical medium to another

Question 2

what are the refractive media in the eyeball (and in what order from out to in)

Answer 2

cornea - aqueous humour - lens - vitreous humour

transparent to allow light to fall on retina

Question 3

how does a sharp image form on the retina

Answer 3

light waves from an object bend at the cornea - bend more at the lens - forms a clear image on the retina - - REFRACTION

Question 4

what needs to happen to allow the eye to focus on an object that has come closer

Answer 4

eye needs more bending power to focus on object - lens becomes thicker and more powerful - clear image is formed on retina again = ACCOMMODATION

*cornea most powerful bender (45D) but lens (15D) has capacity to change its bending power

Question 5

what is the definition of accommodation

Answer 5

the changes occurring in both eyes as it changes focus from a distant object to a close object

Question 6

what are the 3 things that happen simultaneously in accommodation

Answer 6

1. lens changes shape - becomes thicker and more spherical
2. pupil constricts
3. eyes converge

Question 7

what causes the lens to become thicker

Answer 7

ciliary body contraction (parasympathetic)

Question 8

how does the parasympathetic innervation of the ciliary body cause the lens to thicken

Answer 8

innervation - ciliary body contracts - ciliary body bulges - space in the middle decreases - suspensory ligaments become last - lens is no longer under stretch - LENS BECOMES THICKER

Question 9

why do the pupils constrict in accommodation

Answer 9

allows only a few rays from object to pass through - allows sharper focus

Question 10

what causes pupil constriction in accommodation

Answer 10

parasympathetic innervation of the pupillary constrictor muscle

Question 11

what causes the eyes to converge in accommodation

Answer 11

eyes have to turn in to look at close object - extrinsic muscles move eyes - BOTH MEDIAL RECTI

Question 12

what are 4 refractive errors that can occur

Answer 12

1. myopia - short sightedness
2. hyperopia - long sightedness
3. astigmatism - non-spherical curvature of the cornea (or lens)
4. presbyopia - long sightedness of old age

Question 13

what is the effect on vision in myopia

Answer 13

close objects look clear, distant objects appear hazy

Question 14

what is the most common cause of myopia and how does this cause it

Answer 14

eyeball too long - when cornea and lens bend rays of light - form image IN FRONT of retina - far away objects not seen clearly

ie bending power for eye TOO MUCH

Question 15

what are symptoms of myopia

Answer 15

1. headaches
2. divergent squint in infants/preverbal children
3. toddlers - loss of interest in sports/people, more interest in books/picures
4. teachers may notice child losing interest in class

Question 16

how can myopia be corrected

Answer 16

bending power needs to be DECREASED - BICONCAVE lens

• spectacles
• contact lenses
• laser eye surgery

Question 17

what is the effect of hyperopia on vision

Answer 17

close objects look hazy, distant objects appear clear

Question 18

what is the cause of hyperopia

Answer 18

eyeball too short and lens too flat - image of distant image initially formed BEHIND the retina

Question 19

how can people with hyperopia still see distant objects clearly if the image is initially formed BEHIND the retina

Answer 19

person automatically uses accommodative power to make lens thicker - causes image to form on retina

i.e. uses his lens power to see far off things when in normal people no lens power would be used

Question 20

why can people with hyperopia not see close up objects

Answer 20

because they are already accommodating for far away objects - as objects get closer - use more and more power until power is used up

Question 21

what are the symptoms of hyperopia

Answer 21

1. eyestrain after reading/working on computer in young individual
2. convergent squint in children toddlers - need immediate correction to preserve vision in both eyes and prevent lazy eye

Question 22

how can hyperopia be corrected

Answer 22

• BICONVEX glasses - alleviates use of lens for focusing distant objects and “rests” accommodative power
• contact lenses
• laser eye surgery

Question 23

what is the effect of astigmatism on vision

Answer 23

close AND distant objects appear hazy

Question 24

what causes astigmatism

Answer 24

surface of eyeball has different curvatures in different meridian - e.g. like a rugby ball

ie bending of light rays along one taxi never the same as the other axis - image formed is always hazy

Question 25

how can astigmatism be corrected

Answer 25

• special glasses needed - CYLINDRICAL glasses - curved in only one axis
• laser eye surgery
• special contact lenses - topic lenses

Question 26

what causes presbyopia

Answer 26

with age lens gets less mobile/elastic - when ciliary muscle contracts - not as capable as before to change shape - seeing near objects becomes difficult

Question 27

when does presbyopia usually start to occur

Answer 27

in the 5th decade of life

Question 28

how can presbyopia be corrected

Answer 28

correction using BICONVEX reading glasses