9. Refraction by the eye Flashcards
cardinal points - principal plan
o Principal planes intersect the principal axis at right angles at the principal points
factors for principal plane
lens surfaces’ curvature
lens thickness
refractive index
P1 and P2
o Any ray incidences at P1 leaves at P2 at the same vertical distance from the principal axis
N1 and N2
correspond to the centre of a thin lens
o Any ray directed towards N1 leaves as if from N2 parallel and undeviated
cardinal points
in thick lens theory, principal points
do not lie on the surface of the lens
anterior vertrex focal length
distance of the anterior principle focus from the anterior vertex of the lens
PVFL is the same
AVFL and PVFL
do not equal each othe
AVFL reciprocal
AVFL reciprocal in metres = anterior vertex power in dioptres
vertex power and equalivant power
do not equal each other
glasses are graded by
their back vertex power
three major refracting surfaces of the eye
anterior corneal surface and the two surfaces of the lens
refractive index of air
1
refractive index of cornea
1.376
refractive index of aqueous humour
1.336
refractive index of lens cortex
1.638
refractive index of lens core
1.336
refractive index of vitreous
1.336
schematic eye by gullstrand
o Expressed in terms of cardinal points
o Measures in mm behind the anterior corneal surface
o Nodal points are removed from the principal pointsod
nodal points in gullstrand eye
Nodal points straddle posterior pole of human crystalline lens
Pupil of eye only allow small paraxial area of light to enter the eye
These light rays refracted are concentrated on nodal points
Small posterior polar cataract can cause massive visual impairment with small pupil
gullstrand eye
reduced eye by Listing
o Much simpler – treats the eye as a single refracting surface of a power of +58.6D
o Single principal point lies midway between the two principal points of the schematic eye
points in the reduced eye by Listing
o A single nodal point lies midway between the two nodal points
o Nodal point lies in the posterior part of the lens
o Second focal point lies 24.13mm behind the cornea i.e. the retina
listing eye
phakic eye
+58D
aphakic eye
+43D
why is the relative power of the cornea greater
due to the greater difference between air to cornea compared to aqueous / vitreous and lens
construction of a retinal image in a reduced eye
i. Ray passing through the anterior focus (Fa) which after refraction at the principal plane P continues parallel to the axis
ii. Ray passing through the nodal point undeviated
retinal image in a reduced eye
emmetropic eye and accomodation
second principal focus falls on the retina without accommodation
accommodation
ability of the eye to increase its dioptric power
o Most of the change occurs at the anterior lens surface
near point of distinct vision
nearest point at which an object can be seen with maximum accommodation
far point of distinct vision
position of an object such that its image falls on the retina in a relaxed eye (i.e. no accommodation)
- r
o Distance from the fair point from the principal plan
range of accomodation
distance between the far point and near point
amplitude of accomodation
difference in dioptric power of the eye at rest and fully accommodated
static refraction
dioptres power of the resting eye
dynamic refraction
dioptric power of the accommodated eye
Amplitude of accomodation
A = P – R
A: amplitude of accommodation in dioptres
P: near point distance in dioptres
R: far point distance in dioptres
Accommodative power required to foucs a point
A = V – R
A: accommodative power required in dioptres
V: dioptric value of the intermedia point
R: far point distance in dioptres
AC/A
o To maintain binocular single vision – accommodate and converge
o Normal AC/A – 3:1 to 5:1
ways to measure AC/A
Heterophoria method
gradient method
heterophoria method
AC/A = IPD + N (D’ – D)
IPD: inter-pupillary distance in cms
N: working distance in metres
D’: near phoria
D: distant phoria
gradient method
AC/A = (D’-D)/N
D’: near phoria
D: distant phoria
N: working distance in metres
convergence excess esotopria
o Eyes straight for distance, but breakdown into a convergent squint for near
o Can be controlled with bifocal spectacles
