How We See 1 - refraction/accommodation Flashcards
What is refraction?
Bending of light due to transition from travelling through one medium to another
Different speeds through different mediums
Which parts of the eye bend light as it enters?
Cornea and lens
Why does the lens need to change shape to see objects that are closer?
Because when an object is closer only the divergent rays hit the cornea (physics stuff)
Light is not hitting the eye at parallel, it is coming at an angle so the lens needs to change shape to focus it
What bends light more, the cornea or the lens?
The cornea - but it can’t change its shape
What is accommodation? 3 things that occur to accomplish it?
The capacity to change focus from distant objects to close objects
- Lens changes shape
- Pupil constricts
- Eyes converge
When the ciliary muscle contracts what happens?
The ciliary body bulges and and the two halves become closer together
This releases tension of the zonules on the lens and so it becomes thicker
Nerve that innervates the ciliary muscle?
Oculomotor (CN III)
How does the constriction of the pupil contribute to accommodation? Muscle responsible?
When objects are closer the pupil constricts to allow only a few light rays in - increases sharpness of focus
Sphincter pupillae (CN III)
When focusing on something close, what extrinsic muscle of the eye contracts? What does this do to the position of the eye?
- Medial rectus muscle (CN III)
- Causes the corneas of the eyes to converge, rotates the eyeball medially
What are the refractive errors of the eye?
- Myopia - short sightedness
- Hyperopia - long sightedness
- Astigmatism - non-spherical curvature of cornea
- Presbyopia - long sightedness of old age
What is someone with perfect vision called?
An Emmetrope (they have emmetropia)
What is short sightedness? Long sightedness?
Short - can see up close but objects far away are blurry
Long - Can see things far away but up close is blurry
Most common reason for myopia?
The eyeball is too long
Object focuses before it hits the retina
Signs of myopia?
- Divergent squint (eyes don’t converge on close objects)
- Headaches
- Complain of blurry vision far away
What type of glasses correct myopia? How do they do so?
Biconcave glasses
They diverge the light so that it focuses further back in the eye (on the lens)
Possible treatments for myopia/hyperopia?
- Glasses
- Contacts
- LASIK surgery to reshape cornea
Most common reason for hyperopia?
Eyeball is too short
Object focuses behind retina
Signs of hyperopia?
- Eyestrain (discomfort after reading/computer work)
- Convergent squint (can become lazy eye if untreated)
How do biconvex lenses correct hyperopia?
Converge the light rays so they’ll focus sooner
What is astigmatism?
Unequal curvature of the lens or cornea in the horizontal and vertical planes - improperly curved lens/cornea
Type of lenses that correct astigmatism and how?
Cylindrical lenses
Curved more on one side than the other to correct for incorrect curvature in the eye
Type of contacts needed for an astigmatic?
Toric lenses
different because they need to be weighted more on one side so that they rotate to the right orientation when placed in the eye - because improper curvature requires specific lens orientation to correct
What causes presbyopia?
Effect?
Lens becomes less elastic and so can’t change its shape (contract) as much anymore
Nearby objects become hard to see
When does presbyopia usually start? What type of lens corrects it?
Usually begins in 5th decade of life
Corrected by use of biconvex lenses
On a lens prescription what does a negative number indicate? A positive number?
- Negative - myopia
Positive - hyperopia
