9. Refraction by the eye

Question 1

cardinal points - principal plan

Answer 1

o Principal planes intersect the principal axis at right angles at the principal points

Question 2

factors for principal plane

Answer 2

lens surfaces’ curvature
lens thickness
refractive index

Question 3

P1 and P2

Answer 3

o Any ray incidences at P1 leaves at P2 at the same vertical distance from the principal axis

Question 4

N1 and N2

Answer 4

correspond to the centre of a thin lens
o Any ray directed towards N1 leaves as if from N2 parallel and undeviated

Question 5

cardinal points

Answer 5

Question 6

in thick lens theory, principal points

Answer 6

do not lie on the surface of the lens

Question 7

anterior vertrex focal length

Answer 7

distance of the anterior principle focus from the anterior vertex of the lens

PVFL is the same

Question 8

AVFL and PVFL

Answer 8

do not equal each othe

Question 9

AVFL reciprocal

Answer 9

AVFL reciprocal in metres = anterior vertex power in dioptres

Question 10

vertex power and equalivant power

Answer 10

do not equal each other

Question 11

glasses are graded by

Answer 11

their back vertex power

Question 12

three major refracting surfaces of the eye

Answer 12

anterior corneal surface and the two surfaces of the lens

Question 13

refractive index of air

Answer 13

1

Question 14

refractive index of cornea

Answer 14

1.376

Question 15

refractive index of aqueous humour

Answer 15

1.336

Question 16

refractive index of lens cortex

Answer 16

1.638

Question 17

refractive index of lens core

Answer 17

1.336

Question 18

refractive index of vitreous

Answer 18

1.336

Question 19

schematic eye by gullstrand

Answer 19

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

Question 20

nodal points in gullstrand eye

Answer 20

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

Question 21

gullstrand eye

Answer 21

Question 22

reduced eye by Listing

Answer 22

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

Question 23

points in the reduced eye by Listing

Answer 23

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

Question 24

listing eye

Answer 24

Question 25

phakic eye

Answer 25

+58D

Question 26

aphakic eye

Answer 26

+43D

Question 27

why is the relative power of the cornea greater

Answer 27

due to the greater difference between air to cornea compared to aqueous / vitreous and lens

Question 28

construction of a retinal image in a reduced eye

Answer 28

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

Question 29

retinal image in a reduced eye

Answer 29

Question 30

emmetropic eye and accomodation

Answer 30

second principal focus falls on the retina without accommodation

Question 31

accommodation

Answer 31

ability of the eye to increase its dioptric power
o Most of the change occurs at the anterior lens surface

Question 32

near point of distinct vision

Answer 32

nearest point at which an object can be seen with maximum accommodation

Question 33

far point of distinct vision

Answer 33

position of an object such that its image falls on the retina in a relaxed eye (i.e. no accommodation)

Question 34

• r

Answer 34

o Distance from the fair point from the principal plan

Question 35

range of accomodation

Answer 35

distance between the far point and near point

Question 36

amplitude of accomodation

Answer 36

difference in dioptric power of the eye at rest and fully accommodated

Question 37

static refraction

Answer 37

dioptres power of the resting eye

Question 38

dynamic refraction

Answer 38

dioptric power of the accommodated eye

Question 39

Amplitude of accomodation

Answer 39

A = P – R
 A: amplitude of accommodation in dioptres
 P: near point distance in dioptres
 R: far point distance in dioptres

Question 40

Accommodative power required to foucs a point

Answer 40

A = V – R
 A: accommodative power required in dioptres
 V: dioptric value of the intermedia point
 R: far point distance in dioptres

Question 41

AC/A

Answer 41

o To maintain binocular single vision – accommodate and converge
o Normal AC/A – 3:1 to 5:1

Question 42

ways to measure AC/A

Answer 42

Heterophoria method
gradient method

Question 43

heterophoria method

Answer 43

AC/A = IPD + N (D’ – D)
 IPD: inter-pupillary distance in cms
 N: working distance in metres
 D’: near phoria
 D: distant phoria

Question 44

gradient method

Answer 44

AC/A = (D’-D)/N
 D’: near phoria
 D: distant phoria
 N: working distance in metres

Question 45

convergence excess esotopria

Answer 45

o Eyes straight for distance, but breakdown into a convergent squint for near
o Can be controlled with bifocal spectacles