3.4 - Causes and correction of ametropia Flashcards

1
Q

What can spherical ametropia be split into

A

Myopia and hypermetropia

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2
Q

What happens in spherical ammetropia

A

Image formed in wrong place – front or behind retina

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3
Q

What is spherical ametropia caused by

A
  • 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
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4
Q

Causes of ametropia

A
  • 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
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5
Q

What happens when axial length of eye increases by 1mm

A

Eye becomes -2.7 dioptres myopic

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

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6
Q

What is length of cone photoreceptor

A

0.07mm

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7
Q

Where is far point in myopic eye

A

Real point infront of eye

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8
Q

What does light from object point do

A

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

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9
Q

What happens when px looking at faraway object

A

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

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10
Q

What lens is needed to correct myopia

A

Negative

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11
Q

Why is negative lens needed to correct myopia

A

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

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12
Q

Spectacle correction of myopia - ray diagram

A
  • 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
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13
Q

How to fully correct myopes with negative lens

A
  • 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
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14
Q

What lens is needed for spectacle correction of hypermetropia

A

Positive

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15
Q

Spectacle correction of hypermetropia - ray diagram

A
  • 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
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16
Q

What happens in fully corrected eye

A
  • 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
17
Q

When will a sharp image be formed on retina

A

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

18
Q

In hypermetropia where is far point

A

Behind eye

19
Q

What is ocular refraction

A

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

20
Q

What happens to vergence as light travels through space

A

Vergence changes as light travels through space

21
Q

When does vergence not change

A

When parallel rays of light coming from infinity – vergence 0

22
Q

Where is spectacles placed

A

At the vertex distance, d, from the eye

23
Q

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

24
Q

What does spectacle lens produce

A

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

25
What happens in spectacle lens effectivity
Effect of spectacle lens at eye is different for light leaving spectacle lens
26
What happens in myopes in terms of ocular refraction and thin lens power of spectacle lens
Magnitude of the thin lens power of spectacle lens is higher than ocular refraction – because vergence changed when got to eye
27
What happens in hypermetropes in terms of ocular refraction and thin lens power of spectacle lens
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
28
What is meant by effectiivyt
The vergence of light leaving a spectacle lens is different from that striking the eye