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Elkington Optics > Refraction by the eye > Flashcards

Refraction by the eye Flashcards

1
Q

Why is the thin lens formula inadequate to deal with the refracting system of the eye

A

Because it is composed of a number of refracting surfaces separated by relatively long distances

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2
Q

What are the nodal points N1 N2

A

correspond to the centre of a thin lens

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3
Q

what happens to the principal and nodal points when the medium on both sides of the thick lens is the same

A

the nodal points coincide with the principal points.
When the media on opposite sides of the lens are different, the nodal points do not coincide with the principal points

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4
Q

Where do principal points lie on the lens

A

principal points do not lie on the surface of the lens. They may lie within the lens or, in meniscus form lenses, outside the lens substance

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5
Q

What is the back vertex distance

A

the distance between the eye and the back vertex of a spectacle lens

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6
Q

What are the three major refracting surfaces of the eye

A

the anterior corneal surface and the two surfaces of the lens

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7
Q

What is the refractive index of the Air (gullstrand)

A

1.000

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8
Q

What is the refractive index of the cornea (gullstrand)

A

1.376

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9
Q

What is the refractive index of aqueous humour (gullstrand)

A

1.336

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10
Q

What is the refractive index of the lens (cortex-core) (gullstrand)

A

1.386-1.406

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11
Q

What is the refractive index of vitreous humour (gullstrand)

A

1.336

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12
Q

What is the position of cornea (anterior surface in mm behind anterior corneal surface) - gullstrand

A

0mm

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13
Q

What is the position of Cornea, posterior surface in mm (behind anterior corneal surface)- gullstrand

A

0.5mm

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14
Q

What is the position of lens anterior surface behind anterior surface in mm surface- gullstrand

A

3.6mm

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15
Q

What is the position of lens posterior surface behind anterior corneal surface in mm surface- gullstrand

A

7.2mm

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16
Q

What is the position of lens core, anterior surface behind anterior corneal surface in mm surface- gullstrand

A

4.146mm

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17
Q

What is the position of lens core, posterior surface behind anterior corneal surface in mm surface- gullstrand

A

6.565mm

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18
Q

Where is the First principal point p1 located (distance in mm behind anterior corneal surface)- gullstrand

A

1.35mm

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19
Q

Where is the Second principal point P2 located (distance in mm behind anterior corneal surface)- gullstrand

A

1.60mm

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20
Q

Where is the First Nodal point N1 located (distance in mm behind anterior corneal surface)- gullstrand

A

7.08mm

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21
Q

Where is the Second nodal point N2 located (distance in mm behind anterior corneal surface)- gullstrand

A

7.33mm

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22
Q

Where is the First focal point located (distance in mm behind anterior corneal surface)- gullstrand

A

-15.7mm

23
Q

Where is the second focal point located (distance in mm behind anterior corneal surface)- gullstrand

A

24.4mm

24
Q

What is the refractive power of the eye

A

+58.64D

25
Q

Where do the nodal points lie in Gullstrand schematic eye

A

The nodal points straddle the posterior pole of the crystalline lens. The pupil of the eye allows only a relatively small paraxial pencil of light to enter the eye. Such paraxial rays are refracted and concentrated through the nodal points and adjacent posterior lens substance. Therefore even a small posterior polar cataract produces gross impairment of vision when the pupil is small.

26
Q

Where is the Principal point in listing reduced eye (from the anterior corneal surface in mm)

A

1.35mm

27
Q

Where is the Nodal point in listing reduced eye (from the anterior corneal surface in mm)

A

7.08mm

28
Q

Where is the First focal point in listing reduced eye (from the anterior corneal surface in mm)

A

-15.7mm

29
Q

Where is the second focal point in listing reduced eye (from the anterior corneal surface in mm)

A

24.13mm

30
Q

Gullstrand power of lens

A

+19D

31
Q

How is the retinal image constructed in the reduced eye

A

Two rays are used to construct the image formed by parallel light incident upon the eye:
(1) A ray passing through the anterior focus, Fa, which after refraction at the principal plane, P, continues parallel to the principal axis.
(2) A ray passing through the nodal point, N, undeviated.

32
Q

What is the relationship between the retinal image size and the angle subtended by an object at the nodal point

A

Retinal image size is therefore directly related to the angle subtended by an object at the nodal point

33
Q

What would happen if the refractive power of an emmetropic eye was fixed and unalterable

A

only objects at infinity would be clearly seen. Light from nearer objects would be brought to a focus beyond the second principal focus, F2 and no clear image would be formed on the retina

34
Q

How does the eye accommodate

A

increases its dioptric power. The crystalline lens is held suspended under tension by the suspensory ligament which attaches it to the ring of ciliary muscle. Ciliary muscle contraction reduces the tension on the suspensory ligament and lens, allowing the lens to assume a more globular shape

35
Q

Does the lens move anteriorly or posteriorly with accommodation

A

anteriorly

36
Q

Define the far point in relation to accommodation

A

the position of an object such that its image falls on the retina in the relaxed eye, i.e. in the absence of accommodation.
The far point of the emmetropic eye is at infinity.

37
Q

Define the near point in relation to accommodation

A

the nearest point at which an object can be clearly seen when maximum accommodation is used.

38
Q

What is the range of accommodation

A

the distance between the far point and the near point

39
Q

What is the amplitude of accommodation

A

the difference in dioptric power between the eye at rest and the
fully accommodated eye.

40
Q

What is static refraction

A

The dioptric power of the resting eye

41
Q

What is dynamic refraction

A

The dioptric power of the accommodated eye

42
Q

How is the amplitude of accommodation calculated

A

from the reciprocals of the near and far point distances measured in metres

A=P-R
where A is the amplitude of accommodation in dioptres; P is the dioptric value of the near point distance; and R is the dioptric value of the far point distance

43
Q

What is the normal AC/A ratio

A

3:1 to 5:1

44
Q

What is Accommodative Convergence/Accommodation Ratio

A

In order to view a near object, the eyes must not only accommodate to ensure clear retinal images, but they must also converge to maintain binocular single vision

45
Q

What condition results when AC/A ratio is abnormally high

A

convergence excess esotropia may result, in which the eyes are straight for distance but break down to a convergent squint for near

46
Q

What are catoptric/Purkinje Samson images

A

Each refracting interface in the eye also acts as a spherical mirror, reflecting a small portion of the light incident upon it.
Four images are therefore formed by reflection at the four interfaces, the anterior and posterior corneal surfaces and the anterior and posterior lens surfaces. These images are called catoptric images or Purkinje–Sanson images

47
Q

Features of Images 1,2,3 Purkinje images

A

Images I, II and III (from anterior corneal, posterior corneal and anterior lens surfaces respectively) are erect, virtual images because they are formed by convex reflecting surfaces

48
Q

Features of Image 4 Purkinje Samson image

A

real, inverted image because it is formed by a concave reflecting surface

49
Q

Where does Purkinje image 1 lie

A

Behind anterior lens capsule

50
Q

Where does Purkinje image 2 lie

A

Close behind image 1

51
Q

Where does Purkinje image 3 lie

A

In the vitreous

52
Q

Where does Purkinje image 4 lie

A

Concave posterior lens surface

53
Q

Uses of the first image in optics

A

1) Keratometry
2) Diagnosis and management of squint

54
Q

Uses of image 3 and 4 in optics

A

Much information regarding the changes in lens form during accommodation has also been obtained

Elkington Optics (14 decks)

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