Vision: Refraction and Accommodation Flashcards
what is refraction?
light rays bend to form a sharp image in the retina
what is accommodation?
the way we focus on far or near objects by changing how much we bend light rays
what change occurs in the lens as an object comes closer to the eye?
it becomes thicker and more powerful so a clear image is formed on the retina.
what three things happen simultaneously to create accommodation?
> lens changes shape
pupil constricts
eyes converge
describe the process by which the lens thickens?
the ciliary muscle contracts (parasympathetic) making the ciliary body bulge and decreasing the space in the middle. the suspensory ligaments become lax so the lens is no longer under stretch and becomes thicker.
how does the pupils sharpen focus?
it constricts only allowing a few rays to pass through
what innervation does the sphincter pupillae get?
parasympathetic innervation
what is convergence?
when focussing on an object close up our eyes have to turn in
which muscle is used in convergence?
medial rectus, of both sides
which nerve innervate convergence?
CN111 Ocolumotor
what is short sightedness called?
myopia
what is hyperopia?
long-sightedness
what is an astigmatism?
non-spherical curvature of cornea (or lens)
what is presbyopia?
long-sightedness of old age
what emmetrope?
someone with perfect vision
what is the most common cause for myopia?
the eyeball is too long
in myopia where does the image form and what is the effect of this?
in front of the retina so images far away are not seen so clearly
how is myopia corrected?
decreasing the bending power of the eye with biconcave lenses.
what can cause hyperopia?
the eyeball being too short or the cornea and lens being to flat
where is the image of distant objects therefore formed?
behind the retina
why can people with hyperopia not see things close up?
> the person automatically uses his accommodative power to make the lens thicker and form the image on the cornea
since he is using his lens power to see far off things when trying to visual close up things all his power is used up and he cannot focus on them
how is hyperopia corrected?
biconvex lenses focuses distant objects and rests the accommodative power
what are the symptoms of hyperopia?
> eyestrain in working on a computer
> convergent squint in children
what is the cause of astigmatism?
the surface of the eyeball has different curvatures in different meridians so the bending of light rays along one axis will never be the same as that on another axis.
what is the effect of astigmatism on eyesight?
the object always appears hazy at any distance
how is astigmatism corrected?
> laser eye surgery
cylindrical glasses, with only one curved axis
toric contact lenses
what causes presbyopia?
the lens becomes less mobile and elastic with age so it is not capable to change shape when the ciliary muscle contracts
how would you correct presbyopia?
biconvex reading glasses
