3.4 - Causes and correction of ametropia

Question 1

What can spherical ametropia be split into

Answer 1

Myopia and hypermetropia

Question 2

What happens in spherical ammetropia

Answer 2

Image formed in wrong place – front or behind retina

Question 3

What is spherical ametropia caused by

Answer 3

• Axial length of eye incorrect ( myopia - too long, hypermetropia - too short )
• Refractive Power of eye incorrect – due to surface curvature and/ or refractive index have values not compatible with producing images on retina

Question 4

Causes of ametropia

Answer 4

• Failure in coordination during growth of the optical components of the eye
• For high levels of ametropia larger than ±4D, eye is usually too long or too short – axial length
• Sometimes pathology e.g. nuclear cataract - eye changes its refractive state causes scatter of light – refractive index changes – myopic shift

Question 5

What happens when axial length of eye increases by 1mm

Answer 5

Eye becomes -2.7 dioptres myopic

Small changes of axial length even 1mm have significant effect on ocular refraction as eye is powerful

Question 6

What is length of cone photoreceptor

Answer 6

0.07mm

Question 7

Where is far point in myopic eye

Answer 7

Real point infront of eye

Question 8

What does light from object point do

Answer 8

Will strike eye at correct vergence K and form sharp image on retina

Question 9

What happens when px looking at faraway object

Answer 9

Light changes so that it diverges at K at eye and so will form a sharp image on retina

Question 10

What lens is needed to correct myopia

Answer 10

Negative

Question 11

Why is negative lens needed to correct myopia

Answer 11

FOR CORRECTION OF MYOPES NEED A NEGATIVE LENS TO DIVERGE THE INFINITELY INCOMING LIGHT
– Light travelling parallel to axis from infinitely distant object, is diverged by spectacle lens and strikes eye with a divergence of K – value after refraction leaving eye will be K’ – sharp image formed on retina
- Second focal point of spectacle lens, Fsp’ coincides with MR – NEGATIVE LENS REQUIRED
- Rays incident on the eye appear to come from MR and thus form a sharp image on the retina

Question 12

Spectacle correction of myopia - ray diagram

Answer 12

• Light coming into the spectacle enters parallel to the axis and it diverges leaving it
• Extrapolate that ray that leaves the spectacle lens backwards – 2nd focal point
• As that light coming from 2nd focal point of spectacle must diverge at the correct rate so that hits eye with a vergence of K then that point is also far point

Question 13

How to fully correct myopes with negative lens

Answer 13

• The 2nd focal point of spectacle lens must coincide with far point of that eye
• Lens is put in place to make the light appear to come from far point – sharp image formed on retina

Question 14

What lens is needed for spectacle correction of hypermetropia

Answer 14

Positive

Question 15

Spectacle correction of hypermetropia - ray diagram

Answer 15

• Take parallel rays of incoming light from infinitely distant object and converges it
• Follow converging ray and extrapolate it forward, where it crosses the axis is the 2nd focal point of the spectacle lens

Question 16

What happens in fully corrected eye

Answer 16

• 2nd focal point of the spectacle lens must coincide with far point
• If spectace lens achieves that, what eye perceives will come from far point so from sharp image on retina

Question 17

When will a sharp image be formed on retina

Answer 17

Vergence of eye has to be equal to K – ocular refraction

Question 18

In hypermetropia where is far point

Answer 18

Behind eye

Question 19

What is ocular refraction

Answer 19

The thin lens power of a contact lens needed to correct ametropia

Question 20

What happens to vergence as light travels through space

Answer 20

Vergence changes as light travels through space

Question 21

When does vergence not change

Answer 21

When parallel rays of light coming from infinity – vergence 0

Question 22

Where is spectacles placed

Answer 22

At the vertex distance, d, from the eye

Question 23

True or false - spectacle plane refraction will not in general be the same as ocular refraction

Answer 23

True

Question 24

What does spectacle lens produce

Answer 24

An ‘effective power’ that varies at different distances from the eye

Question 25

What happens in spectacle lens effectivity

Answer 25

Effect of spectacle lens at eye is different for light leaving spectacle lens

Question 26

What happens in myopes in terms of ocular refraction and thin lens power of spectacle lens

Answer 26

Magnitude of the thin lens power of spectacle lens is higher than ocular refraction – because vergence changed when got to eye

Question 27

What happens in hypermetropes in terms of ocular refraction and thin lens power of spectacle lens

Answer 27

Hypermetropes │Fsp│ is less than │K│ - power of spectacle lens is less than ocular refraction.

E.g. light might leave spectacle lens correcting hypermetrope at +2.00D, but will strike eye at +2.1D as its converged slightly more by the time its reached eye

Question 28

What is meant by effectiivyt

Answer 28

The vergence of light leaving a spectacle lens is different from that striking the eye