3.2 - Emmetropia and Myopia

Question 1

What is emmetropia

Answer 1

Sharp optical image formed at retina

Second focal point of eye Fe’ coincides with the retina for an unaccommodated eye

Question 2

Where is far point located in emmetropic eye

Answer 2

At infinity

Question 3

Where does 2nd focal point in emmetropic eye fall on

Answer 3

Retina

Question 4

What is meant by unaccommodated

Answer 4

Distance vision - eye hasn’t increased power in an attempt to focus at something that’s close

Question 5

What happens in ray diagram for emmetropic eye

Answer 5

Parallel rays entering eye = relate to infinitely distant object = converged by eye = single surface reduced eye to a focal point which is at retina itself

Question 6

Can VA be improved with optical aids

Answer 6

Visual acuity can’t be improved with optical aids e.g. contact lenses or glasses but it doesn’t have to be 6/6

Question 7

Does optical correction stop at 6/6

Answer 7

NO..
You can use optical correction and create a VA of 6/5 or 6/4 – becomes end point or point where refracted error fully corrected
Shouldn’t stop at 6/6 – try correct all of refractive error – the point at which no optical correction can make any further improvement ( no such thing as Snellen standard observer 6/6 )

Question 8

What is far point

Answer 8

Object point conjugate with the retina in the unaccommodated eye
Anything further than far point will not be in focus

Question 9

What is conjugate pairs - object and image

Answer 9

Object point will form image = object and image points = points conjugate are in pairs

Question 10

If at object point is at infinity where is image formed

Answer 10

At 2nd focal point = object conjugate with 2nd focal point

Question 11

What is standard emmetropic reduced eye

Answer 11

Single surface model of eye - reduced eye

Question 12

What is power of standard emmetropic reduced eye

Answer 12

+60D

Question 13

What is n of standard emmetropic reduced eye

Answer 13

1

Question 14

What is n’ of standard emmetropic reduced eye

Answer 14

4/3

Question 15

What is k

Answer 15

Far point distance measured from the surface of the reduced eye

Question 16

What is K

Answer 16

Ocular refraction.

It is the vergence of light striking before eye

Question 17

What is k’

Answer 17

Axial length of the reduced eye - distance from single surface of eye to retina

Question 18

What is K’

Answer 18

Dioptric length of eye

The vergence of light leaving the surface of the reduced eye that forms an image at the retina.

Question 19

What is Fe

Answer 19

(Equivalent) power of the reduced eye

Question 20

What is Fsp

Answer 20

Thin lens power of a spectacle lens

Question 21

Where is far point distance k for emmetropic reduced eye

Answer 21

At infinity - infinitely distant for emmetropic reduced eye

Question 22

What is ocular refraction K equation

Answer 22

K = n/k

Question 23

What is dioptric length K’ equation

Answer 23

K'= n'/k'
K' = K + Fe

Question 24

Why is ocular refraction K 0 for emmetropic reduced eye

Answer 24

K = n/k
n is 0
k is infinity 
Anything divided by infinity is 0
So K is 0

Question 25

What is vergence of light travelling from infinitely distant object

Answer 25

0

Question 26

What is myopia

Answer 26

Second focal point Fe’ falls in front of the retina for an unaccommodated eye
Retinal image blurred – vision reduced as light spreads out by the time it hits retina

Question 27

Is myopia long or short sightedness

Answer 27

Short sightedness

Question 28

Where is far point located in myopic eye

Answer 28

In front of the eye – a real point

Far point - object point conjugate with retina

Question 29

What happens in myopic eye

Answer 29

Objects closer to the unaccommodated eye are seen clearly
Distant objects might focus short to retina but its possible to find distance closer to eye - produce sharp image on retina - furthest distant this can be is far point

Question 30

Where is 2nd focal point for myopic eye

Answer 30

Infront of retina

Question 31

What happens too axial length in myopic eye

Answer 31

Its too long

Question 32

What is far point - more detail

Answer 32

Distant objects might focus short to retina but its possible to find distance closer to eye - produce sharp image on retina - furthest distant this can be is far point
e.g far point was 50cm in front of eye - can find object position at 50cm = produce sharp image on retina - anything beyond 50cm wouldn’t be sharply focused

Question 33

What happens if far point was 50cm in front of eye i.e. object position at 50cm

Answer 33

Produce sharp image on retina - anything beyond 50cm wouldn’t be sharply focused BECAUSE
Anything further than far point will not be in focus

Question 34

How to find axial length k’

Answer 34

1. k = given in question
2. K: K = n/k
3. K’: K’ = K + Fe
4. k’: K’ = n’/k’ SO k’ = n’/K’

Question 35

What does it mean if k is -0.2m

Answer 35

Far point 0.2m to left of eye ( left because its negative )

Question 36

What is ocular refraction, K

Answer 36

• Incident vergence required for a sharp optical image to be formed on the retina
• The thin lens power of a contact lens needed to correct ametropia
• Change in power of cornea if refractive surgery

Question 37

What is K equal to

Answer 37

The power of a thin lens placed in contact with the eye to fully correct its ametropia

Question 38

Where will object be at distance k

Answer 38

At far point location

Question 39

What are K and K’

Answer 39

• K and K’ are parameters/properties of reduced eye and not always vergences from ray tracing
• They are not necessarily object or image locations

Question 40

When might vergences L and L’ equal K and K’

Answer 40

If the image is formed on the retina and the object is at the far point