Optics of the eye Flashcards
What are the 6 different refracting surfaces of the eye
Front and back of cornea
Front and back of anterior lens
Front and back of posterior lens
What is the optical axis of the eye
The straight line passing through the geometrical cetner of a lens, joining the two centres of curvature of its surfaces
What is the visual axis of the eye
A line passing from the fovea through the nodal point of the eye
What is Gullstrand’s simplified Schematic eye
Consists of a single refrating surface and thin lens
Equivalent power of +59.7 D in the relaxed state
What is the Reduced schematic eye
Assumes the eye as one refractive surface with a refractive index of n= 1.333
What are the characteristics of the reduce schematic eye
2 principal points that conincide with each other and with the vertex of the surface
2 nodal points coincide with each other and with the centre of curvature of the surface
Optical axis becoem the visual axis
Hypothetical pupil lies at the refracating surface
Enterance and exit pupils coincide with hypothetical pupil
What are the parameters of the standard reduced emmetropic eye
Reduced power = +60D
Axial length of eye k’ = +22.22 mm
Refractive index of eye= 1.333 (n’ when calculating power)
What are the 4 types of refractive erros
Emmetropia
Myopia
Hyperopia
Astigmatism
What is far point
The point farthest from the eye at which an object is accurately focused on the retina when the accomdation is completely relaxed
What is near point
The point nearest the eye at which an object is accrately focused on the retina at full accomodation is called near point
What is emmetropia
When is an unaccomodated eye, te parallel pencils of rays from a distsant obj to a sharp focus on the retina
The far point is at infinity and secondar focal point coincides with fovea
How is near point (accomodation determined)
By age:
Esstimated accommodation = 15 - (age/4)
What is ametropia
Having a refractive error which results in imbalance between power of the eye and its axial length
What is axial amertropia
Assummued to have standard eye power = +60D
Refractive error attributd to error in axial length
What is refractive ametropia
Axial legnth of reduce eye is assumed its standard value = 22.22mm
Defect attributed to an eror in power
What is myopia
rays from a distant object point focus in front of the macula in the unaccommodated eye
Optical system too powerful for its axial length/ axial length too long
What is the sign of the far point in myopia
Negative as it is in front
How is myopia corrected
Negative, concanve lense
What is hyperopia
rays from a distant object point focuses behind the macula in unaccommodated eye
Eye not powerful enough, axial length too short
What is the sign of the far point in hyperopia
Posistive, as far point is behind the eye
How is hyperopia corrected
Posistive, convex lenses
What is astigmatism
Two focal lines image formed at different ditance generally perpendicular to each other for a point object
Eyeball is not round thus the horizontal and vertical axis have different focal points
How is astigmatism corrected
Sphero-cylindrical lenses
What isthe circle of least confusion (astigmatism)
Dioptric midway point betwen the 2 focal planes, cross sction of which is circular
What is the interval of sturm
dioptric or physical distance between the 2 focal lines
What is Presbyopia
Associated with the againg of eye that results in progressively worsenening ability to focus clearly on close objects
Crystallne lenses losing its flexibilty as we get older
How is presbyopia corrected
Posistive, convex lenses
