Refraction by the eye Flashcards
Why is the thin lens formula inadequate to deal with the refracting system of the eye
Because it is composed of a number of refracting surfaces separated by relatively long distances
What are the nodal points N1 N2
correspond to the centre of a thin lens
what happens to the principal and nodal points when the medium on both sides of the thick lens is the same
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
Where do principal points lie on the lens
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
What is the back vertex distance
the distance between the eye and the back vertex of a spectacle lens
What are the three major refracting surfaces of the eye
the anterior corneal surface and the two surfaces of the lens
What is the refractive index of the Air (gullstrand)
1.000
What is the refractive index of the cornea (gullstrand)
1.376
What is the refractive index of aqueous humour (gullstrand)
1.336
What is the refractive index of the lens (cortex-core) (gullstrand)
1.386-1.406
What is the refractive index of vitreous humour (gullstrand)
1.336
What is the position of cornea (anterior surface in mm behind anterior corneal surface) - gullstrand
0mm
What is the position of Cornea, posterior surface in mm (behind anterior corneal surface)- gullstrand
0.5mm
What is the position of lens anterior surface behind anterior surface in mm surface- gullstrand
3.6mm
What is the position of lens posterior surface behind anterior corneal surface in mm surface- gullstrand
7.2mm
What is the position of lens core, anterior surface behind anterior corneal surface in mm surface- gullstrand
4.146mm
What is the position of lens core, posterior surface behind anterior corneal surface in mm surface- gullstrand
6.565mm
Where is the First principal point p1 located (distance in mm behind anterior corneal surface)- gullstrand
1.35mm
Where is the Second principal point P2 located (distance in mm behind anterior corneal surface)- gullstrand
1.60mm
Where is the First Nodal point N1 located (distance in mm behind anterior corneal surface)- gullstrand
7.08mm
Where is the Second nodal point N2 located (distance in mm behind anterior corneal surface)- gullstrand
7.33mm
Where is the First focal point located (distance in mm behind anterior corneal surface)- gullstrand
-15.7mm
Where is the second focal point located (distance in mm behind anterior corneal surface)- gullstrand
24.4mm
What is the refractive power of the eye
+58.64D
Where do the nodal points lie in Gullstrand schematic eye
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.
Where is the Principal point in listing reduced eye (from the anterior corneal surface in mm)
1.35mm
Where is the Nodal point in listing reduced eye (from the anterior corneal surface in mm)
7.08mm
Where is the First focal point in listing reduced eye (from the anterior corneal surface in mm)
-15.7mm
Where is the second focal point in listing reduced eye (from the anterior corneal surface in mm)
24.13mm
Gullstrand power of lens
+19D
How is the retinal image constructed in the reduced eye
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.
What is the relationship between the retinal image size and the angle subtended by an object at the nodal point
Retinal image size is therefore directly related to the angle subtended by an object at the nodal point
What would happen if the refractive power of an emmetropic eye was fixed and unalterable
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
How does the eye accommodate
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
Does the lens move anteriorly or posteriorly with accommodation
anteriorly
Define the far point in relation to accommodation
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.
Define the near point in relation to accommodation
the nearest point at which an object can be clearly seen when maximum accommodation is used.
What is the range of accommodation
the distance between the far point and the near point
What is the amplitude of accommodation
the difference in dioptric power between the eye at rest and the
fully accommodated eye.
What is static refraction
The dioptric power of the resting eye
What is dynamic refraction
The dioptric power of the accommodated eye
How is the amplitude of accommodation calculated
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
What is the normal AC/A ratio
3:1 to 5:1
What is Accommodative Convergence/Accommodation Ratio
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
What condition results when AC/A ratio is abnormally high
convergence excess esotropia may result, in which the eyes are straight for distance but break down to a convergent squint for near
What are catoptric/Purkinje Samson images
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
Features of Images 1,2,3 Purkinje images
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
Features of Image 4 Purkinje Samson image
real, inverted image because it is formed by a concave reflecting surface
Where does Purkinje image 1 lie
Behind anterior lens capsule
Where does Purkinje image 2 lie
Close behind image 1
Where does Purkinje image 3 lie
In the vitreous
Where does Purkinje image 4 lie
Concave posterior lens surface
Uses of the first image in optics
1) Keratometry
2) Diagnosis and management of squint
Uses of image 3 and 4 in optics
Much information regarding the changes in lens form during accommodation has also been obtained
