Refraction and Accommodation Flashcards
Define Refraction
the bending of light when it passes from one optical medium to another to form sharp image on retina
Describe the effect of convex and concave lenses
Convex - bend light together
Concave - bends light away
What parts of the eye refract light
Cornea
Aqueous humor
Lens
What part of the light has the greatest bending power, and why
Cornea - 45D
Biggest difference in objects e.g. air - eye
The lens has a power of 15D but has the capacity to do what
Change its bending power
What happens to the lens when an objects distance decreases
Image rays are more divergent therefore lens needs to become spherical/thicker which increases the bending power so image can reach retina
Define accommodation
The ability to change focus from far off to near objects by changing how much we bend the light rays
What are the three ways the eye does accommodation
Lens thickening
Pupil constricts
Eyes converge
How does accommodation occur in the lens
In close objects the suspensory ligaments relax so lens is no longer being pulled therefore lens flops down becoming thicker and more spherical so can focus clearly on close objects
What innervates the lens accommodation
parasympathetic
How does accommodation of the pupils occur
In close objects the pupils constricts tallow only a few rays from object to pass through pupil
What muscle controls pupil accommodation
Spinchter pupillae
What innervates the spinchter pupillae
parasympathetic
What muscle is used in eyes converging in accommodation
Medial rectus in both eyes helps focus on close up objects as eyes converge to look at it
Why is the medial rectus shorter and ticker than the lateral
We use the medial rectus alot more than the lateral
What innervates the medial rectus for accommodation
Oculomotor nerve - IIIN
What is the name for perfect vision
emmertopia
What is 4 refractive errors
Myopia
Hyperopia
Astigmatism
Prebyopia
Define myopia
Short-sightedness
Distant objects not seen as clear
What occurs in myopia
Image is formed infant of retina as cornea and lens bends the rays to much for the eyeball
What effect does myopia have on close objects
When objects brought close the rays diverge and the bending power is useful as image is formed on retina without much curvature from lens
What is the most common cause of myopia
Long eye ball
What is the symptoms of myopia
Headaches
Divergent squint (infants, toddlers)
Loss of interest in school, sports etc
Complaining of not seeing distant objects
How is myopia corrected
Bending the light ray less so can focus on retina
Possible though biconcave lenses: spectacles, contacts
Laser eye surgery
How does laser eye surgery work in correcting myopia
Flatten out stroma and reshape cornea decreasing power
Define hyperopia
Long sightedness -close objects not seen as clear
What occurs in hyperopia
Image is formed behind the retina as use up accommodation power to objects far away, making lens thicker, so when looking at objects up close they appear hazy as al lens power has been used up
What is the common cause of hyperopia
Eyeball to short or cornea
Lens to flat
What is the symptoms of hyperopia
Convergent squint (children, toddlers)
eyes strain after reading
How is hyperopia corrected
Make lens fatter so can bend light rays more and focus on the retina
What is used to help correct hyperopia
Biconvex glasses - focus on distant objects and rest accommodative power
contact lenses
Laser eye surgery
Define Astigmatism
Non spherical curvature of cornea so close and distant object appear hazy
What occurs in astigmatism
Surface has different curvature in different merdians so the bending of light rays along one axis will never be the same as another axis therefore image formed is hazy no matter the difference
How is astigmatism corrected
Cylindrical (special) glasses - curved only in one axis
Toric lenses
Laser eye surgery
Define presbyopia
Long sightedness occurring in old age (5th decade)
What occurs in presbyopia
With age the lens fibres are less elastic so can no longer to floppy movement as well so when ciliary muscles contract its not as capable to change shape
How is presbyopia corrected
Biconvex - reading glasses
What is OD and OS
OD - Right eye
OS - Left eye
What is CYL and when is it prescribed
Cylinder correction
prescribed for astigmatism
What is also included with CYL
AXIS
What does SPH stand for and when is it prescribed
Spherical contraction - the amount of lens power
Myopia/hyperopia
What is BC and DIA
Base curve 8-10cm
Diameter of lens
