How we see Flashcards
What is refraction?
light rays bend to form a sharp image on the retina
What is accommidation?
We can focus on far or near objects by changing how much we bend the light rays
This process is achieved by the lens changing its shape
What are refractive errors?
simetimes there is a mismatch between how much we bend light rays leading to refractive errors
What is refraction?
Bending of light when it passes from one optical medium to another
(An optical medium is material through which electromagnetic waves propagate)
How does a sharp image form on the retina?
light waves form an object bend at the cornea, bend some more at the lens to form a clear image on the retina and this bending of light is called refraction
What happens when an object comes closed to the eye?
when an object comes clsoer to the eye, the eye needs more bending power to focus on an object so the lens becomes thickers and thereofre more powerful, and a clear image is formed on the retina again
What things in your eye bend light rays?
The cornea is the most powerful “bender” of light (45D) but lens (15D) has the capacity to change its “bending power”
Our eyes have the ability to change focus form distant objects to close objects
the changes are occuring in both eyes as it changes focus form a distant to a close object
What is this called?
accomidation
what are the 3 things that happen all togther and make up accomidation?
lens change shape (become thicker and more spherical)
pupil constricts
eyes converge
What is the process of the lens thickening in accomidation?
the ciliary body contracts (parasympathic - CN III) causing the space inbetween the ciliary body and the lens to decrease and the suspensory ligamens to become lax meaning the lens is no longer streched leading to the lens thickening and becoming more spherical
a thicker lens is more powerful and can focus on close objects
When we look at an object close up we need a sharp focus so how is the focus sharpened?
pupil constricts to only allow a few rays to pass through
What is responsible for constricting the pupil?
pupillary constrictor (sphincter pupillae) is a concentric muscle around the border of the pupil which gets parasympathetic innervation
What does it mean when the eyes converge as part of accomidation?
when focusing on a object close up, our eyes have to turn to look at the object
What muscles do we use to converge our eyes?
medial rectus muscles of both eyes
(IIIn on both sides)
How do humans medial recti msucles compare to their lateral recti muscles?
Humans spend a lot of time doing ‘close’ work, so our Medial recti muscles are thicker than our lateral recti muscles
What are 4 common refractive errors?
myopia
hyperopia
astigmatism
presbyopia
What is prefect vision called?
emmetrope, i.e. you have emmetropia
What is myopia?
short sightedness
struggle to see things that are far away
What is the most common cause of myopia
eyeballs are too long
Explain the cause myopia?
It occurs when the shape of your eye causes light rays to bend (refract) incorrectly, focusing images in front of your retina instead of on your retina
So when the cornea + lens bend rays of light, they make the image form IN FRONT OF the retina so far away objects not seen clearly
(Basically the “bending power” of cornea + lens is too much for that eyeball)
In a myopic person, can they see closer up objects clearly?
Yes
When the object is brought closer, divergent rays from the object reach the cornea and then this “bending power” comes to use. The image is formed on the retina without needing to increase curvature of lens (i.e. without using the accommodative power)
Can see near things clearly i.e. near-sighted
What symptoms may be seen in myopia?
headaches, complain of not being able to see the blackboard/distant objects
infants/preverbal children - divergent squint
toddlers - loss of intrest in sports/people and mor eintrest in books/pictures
teacher may notice child loosing interest in class
Why may someone with myopia develop a divergent squint?
Normally eyes converging when using accommodating power but in myopics they don’t use accommodating power so eyes may divert
How can myopia be corrected?
bending power of the eyes need to be decreased and this is done by using biconcave lenses
e.g. spectacles, contact lenses and laser eye surgery
What is hyperopia?
long-sightedness
struggle to see things up close but far objects are seen normally
opposite of muopia
What causes hyperopia?
Often it occurs when the eyeball is too short, or the lens or cornea is too flat so the image of a distant object is formed behind the retina
The person then automatically starts to use his accommodative power and makes the lens thicker. This causes the image to form on the retina.
So he is using his lens power to see far off things (that he should normally be seeing without using any power)
When seeing closer objects, he uses more and more power until ultimately, his power is all used up!
So he can’t see nearby objects ie. “longsighted”
What are the symptoms of hyperopia?
Symptoms of eyestrain after reading/working on the computer in a young individual
Convergent squint in children/ toddlers - needs immediate correction with glasses/lenses to preserve vision in both eyes and prevent a “lazy eye”
How can hyperoipa be corrected?
Biconvex glasses alleviates use of glasses for focussing distant objects and ‘rests’ the accomodative power
Contact lenses
Laser Eye surgery
What is astigmatism?
non-spherical curvature of cornea (or lens)
an imperfection in the curvature of your eye’s cornea. This hinders your eye’s ability to focus light onto your retina, resulting in blurred and distorted vision
What is the cause of astigmatism?
a persons lens or cornea has an irregular curve so the bending of light rays along one axis will never be the same as that of the other axis
this leads to the image formed always being hazy no matter what distance the object is
How is astigmatism corrected?
So to correct it we need special glasses – called cylindrical glasses (which are curved in only one axis)
Laser eye surgery can also be used to correct the defect
Need special contact lenses called toric lenses
What is presbyopia?
long-sightedness of old age
What causes presbyopia?
With age the lens gets less mobile/elastic
So when the ciliary muscle contracts, it is not as capable as before to change shape
So seeing near objects/reading the newspaper starts to become difficult - needs glasses to read
When would someone most commonly experince presbyopia?
Usually starts in 5th decade of life
How is presbyopia treated?
Correction is by using biconvex “reading glasses”
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