3: refractive surface and vergence

using model eyes to learn about refractive surface

top 2 show 2 surfaces of the cornea

bottom simplified - lenses is taken away and its power is assumed ot be contained within the cornea 

power of the eye is around 60 D

40 D contained within the cornea  

reduced eye

a simpler model eye which has only 1 refracting surface

all dioptric power is contained within conreas as theres no lens

only 4 dimensions - only 4 numbers

radius of curvature of cornea = 5.55mm axial length 22.22mm

if radius of curvature is smaller the power increases

refractive index within : 4/3. power of reduced eye is 60 D

if a patient can read the smallest line on a snellen chart and get it all correct does that mean theyre emmetropic

no, because the might have been accomodating

refractive error equation

4/3 = refractive error ininsde the eye

4/3 - n (1.00) / 5.55mm = 60 D

distance vision, vergence and the dioptre 

huyens principle for the spreading of light 

1 is more curved than the second

gets flatter as it moves away from the source

wavefront curvature reeuces with increased distance from the source

distant vision

wavefront is flat

infinite radius - means 0 curvature

vergence of light is also 1/radium in m

unit for vergeance is the dioptre

when radius is large, ie. infinity it will be 0

1/ infinity is 0

so light from distant target has 0 vergence

vergence 

denotes the divergence of light travelling from an object or the convergence of light tracelling to an image

  vergence is the recoprocal of an object distance in metres , or image distance multiplied by the refractive index of corresponding medium

e.g when object is in air vergence of light from an object is just 1/ distance from object 

minus distance if diverging plus if converging 

unit of vergeance is dioptre  

what is a dioptre

a unit proposed by monoyer to evaluate the refractive power of a lens or optical system

power of the lens usually denoted by the symbol F - measured in D

Fe is used for the power of the eye + or -

value in dioptres must always have a sign + or -

a lens/optical system with a +ve power makes incoming light more convergent

lens with -ve makes incoming light more divergent

paraxial equation

vergence of light after refraction = vergence of light as it meets the lens (or optical system) added to the power of the lens (or optical system)

e.g if divergent light (vergence -3D) strikes a converging lens with power +10 D, after refraction the vergence is +7D

more on vergence 

converging rays have a + ve vergence 

diverging rays have a -ve vergence 

parallel rays of light have 0 vergence as 1/infinity is 0

rays of light are only parallel when they come from a distant target 

we use 6m testing distance as its closet to 0

distance vision 

from 6m, rays of light are still diverging when they arrive at the eye 

but the divergence is low 

low enough that the rays can be considered parallel 

3m room and a mirror

can also be done via a mirror

have patient sat 3m from mirror and test letters above the patient

3m to the mirror and 3m from mirror back which adds to 6m

lateral inversion

issue using a mirror

letters are back to front