Vision 1 - refraction and accommodation Flashcards
what is refraction
bending of light as it passes from one optical medium to another
what are the refractive media in the eyeball (and in what order from out to in)
cornea - aqueous humour - lens - vitreous humour
transparent to allow light to fall on retina
how does a sharp image form on the retina
light waves from an object bend at the cornea - bend more at the lens - forms a clear image on the retina - - REFRACTION
what needs to happen to allow the eye to focus on an object that has come closer
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
what is the definition of accommodation
the changes occurring in both eyes as it changes focus from a distant object to a close object
what are the 3 things that happen simultaneously in accommodation
- lens changes shape - becomes thicker and more spherical
- pupil constricts
- eyes converge
what causes the lens to become thicker
ciliary body contraction (parasympathetic)
how does the parasympathetic innervation of the ciliary body cause the lens to thicken
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
why do the pupils constrict in accommodation
allows only a few rays from object to pass through - allows sharper focus
what causes pupil constriction in accommodation
parasympathetic innervation of the pupillary constrictor muscle
what causes the eyes to converge in accommodation
eyes have to turn in to look at close object - extrinsic muscles move eyes - BOTH MEDIAL RECTI
what are 4 refractive errors that can occur
- myopia - short sightedness
- hyperopia - long sightedness
- astigmatism - non-spherical curvature of the cornea (or lens)
- presbyopia - long sightedness of old age
what is the effect on vision in myopia
close objects look clear, distant objects appear hazy
what is the most common cause of myopia and how does this cause it
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
what are symptoms of myopia
- headaches
- divergent squint in infants/preverbal children
- toddlers - loss of interest in sports/people, more interest in books/picures
- teachers may notice child losing interest in class
how can myopia be corrected
bending power needs to be DECREASED - BICONCAVE lens
- spectacles
- contact lenses
- laser eye surgery
what is the effect of hyperopia on vision
close objects look hazy, distant objects appear clear
what is the cause of hyperopia
eyeball too short and lens too flat - image of distant image initially formed BEHIND the retina
how can people with hyperopia still see distant objects clearly if the image is initially formed BEHIND the retina
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
why can people with hyperopia not see close up objects
because they are already accommodating for far away objects - as objects get closer - use more and more power until power is used up
what are the symptoms of hyperopia
- eyestrain after reading/working on computer in young individual
- convergent squint in children toddlers - need immediate correction to preserve vision in both eyes and prevent lazy eye
how can hyperopia be corrected
- BICONVEX glasses - alleviates use of lens for focusing distant objects and “rests” accommodative power
- contact lenses
- laser eye surgery
what is the effect of astigmatism on vision
close AND distant objects appear hazy
what causes astigmatism
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
