Chapter 49 Flashcards
What do convex lenses do to light passing through? How about concave lenses?
- Convex lenses converge light rays onto a single point (the focal point)
- Concave lenses diverge light rays
What is the definition of a focal point?
the distance beyond a convex lense at which parallel ray converge to a common point
What is the formula for determining focal point?
1/f = 1/a + 1/b
f= focal length
a= distance of the light point source
b= distance of the focal point on the other side of the lens
What is a diopter a measue of?
-refractory power
What is the equation to calculate diopters?
Diopter= 1meter / focal length
-e.g. focal length of .5m has a diopter of +2 and 0.1 has a diopter of +10
When is a diopter a negative value?
- When the focal point is in front of the lens (concave lenses).
- e.g. is concave cylindrical leses
In a relaxed state, what is the shape of the lense?
Flattened
What is the role of suspensory ligaments in the control of lens shape?
- Attach to the anterior border of the choroid and retina
- pull the lens edges towards the outer circle of the eyeball
What are the two sets of ciliary muscles?
- meridional
- circular fibers
What do the meridional muscles cause when they contract?
-They pull the insertion of the suspensory ligament closer to the origin and release tension on the lens
What is the role and action of the circular portion of the ciliary muscles?
- act like a sphincter decreasing the diameter of the circle of ligaments
- again releasing tension on the ligaments and lens
How is the ciliary muscle controlled?
-almost entirely by parasympathetics riding on CNIII
What is presbyopia? What causes it?
- a progressive inability to focus on near objects
- caused by a progressive thickening and inelasticity of the lens which, in old age, eventually leads to the inability of the lens to change shape at all (0 diopters)
What are the two roles of the pupil?
- control the amount of light entering the eye
- increase depth of focus by constricting
What is the name for ‘normal’ vision?
-emmetropia
(from greek emmetro meaning ‘well-fiting’)
What does it mean to have emmetropic vision?
Means that distant parallel light rays are focused sharply on the retina when the ciliary muscle is completely relaxed
What condition is known as far-sightedness? What are some characteristics?
- hyperopia
- caused by an eyeball that is too short for the lens system or a lens that is too weak
- parallel light rays are focused behind the retina
Can a person correct for hyperopia?
- yes
- the lens can relax (accomodate) to bring the focal point onto the retina
What is near-sightedness? What are some characteristics?
- Myopia
- eyeball that is too long or a lens that has too much refractive power when relaxed
Can a person with myopia compensate for their deficit?
- no
- a person with myiopia has a definite ‘far point’ beyond which they are unable to focus
How does one correct for myopia?
By placing a concave lens infront of the eye to diverge the incoming rays before they enter the eye.
What is a stigmatism? How is it corrected?
- When light rays passing through an eye have different focal points at perpendicular planes.
- with AWESOME physics: First the focal point of one plane is determined using a set of black bars to focus on (looks like a clock), then that is held constant while the other plane is determined and corrected for.
What is a cataract?
-denatured proteins which form a cloudy or opaque area in the lens
Hoe are cataracts corrected?
-surgical removal and replacement with either a plastic implant lens or very powerful convex lenses
