Visual Defects - Garner Flashcards
What is the fovea?
Fovea is the point of highest visual acuity in the retina where light can reach the photoreceptors directly
What are the process of image formation by the eye?
1) Refraction by the cornea
2) Refraction by lens
3) Lens accomodation
what is lens accomodation?
rounding of the lens increases the curvature of the lens surface so increases the refractive power. The ability to accommodate the lens changes with age.
The lens changes shape by the contraction of the ciliary muscles.
What is Hyperopia?
Hyperopia = farsightedness
DISTANT OBJECTS CAN BE SEEN CLEARLY
This happens when the eyeball is too short from front to back so light rays are focused some point behind the retina.
So the retina sees a blurry circle.
How are lenses used to correct Hyperopia (farsightedness)?
HYPERPIA = FARSIGHTEDNESS (DISTANT OBJECTS CAN BE SEEN CLEARLY)
Accomodation of the lens is needed for distant objects, and near objects cannot be brought into focus.
A convex lens is placed in front of the eye to provide the necessary refraction to allow near objects to be brought into focus.
What is Myopia?
MYPOIA = nearsightedness
(short distance seen clearly)
This occurs when the eyeball is too long from front to back.
When looking at objects far away the parallel light rays converge at some point in front of (before)the retina.
As a result the retina sees an unfocused blurry circle.
How are lenses used to correct Myopia (nearsightedness)
MYOPIA = nearsightedness
(can see short distance clearly)
A Concave lens is placed in front if the eye to provide necessary refraction to allow distant objects to be brought into focus on the retinal surface.
What happens in a photorefractive keratectomy?
This is corrective laser surgery. A laser is used to reshape the cornea and increase or decrease the amount of refraction possible.
what type of photreceptor detects colour and works best in daylight?
Cones - colour system, daylight.
Rods - No colour, low light levels
What type of photoreceptor has a lower sensitivity to light?
Cones (lower sensitivity) - work in daylight and detect colour.
Rods - High sensitivity to light (specialised for night vision)
What photoreceptors provide high acuity vision?
Cones provide high acuity vision, they are concentrated in the fovea.
Rods provide low acuity vision (not present in fovea) (provide night vision)
What is young-helmholtz trichromatic theory?
At each point in the retina there is a cluster of 3 receptor types. Each being sensitive to either blue, green or red. The mixing of these receptors gives rise to different colours.
What is the term for normal colour vision?
trichromat
What is the term for alternate perception of colour?
anomalous trichromat
What is the term for colour blindness?
Dichromat
