Chapter 9  (Refraction by the eye)

1. In the schematic eye of Gullstrand:

a. the human model eye is based on the principal of thick lenses

b. the eye is about 24.0 mm in axial length

c. the nodal points lie on either side of the posterior surface of the lens

d. the nodal points coincide with the principal points

e. the cornea contributes 2/3 to the power of the eye
 

Answera.T   b.T   c.T   d.F   e.T
The schematic eye of Gullstrand is based on the principal of thick lenses. The eye has an axial length of 24.00mm. The principal planes are located 1.35mm and 1.60mm from the corneal apex; nodal points are located 
7.08mm and 7.33mm from the corneal apex, and thus straddle the back surface of the lens. The complete eye has a refractive power of 58.64, cornea contributes 43.05D and the lens 19.11D.
 
 

2. In the reduced eye of Listing:

a. the refractive power is stronger than that of the schematic eye of Gullstrand

b. the whole eye is regarded as a single refractive surface

c. the second focal point lies on the retina

d. the nodal point lies at the posterior surface of the lens

e. the principal plane lies at the anterior surface of the lens
 

Answera.F   b.T   c.T   d.T   e.F
The power is the same for the reduced eye and the schematic eye.
In Listing's reduced eye, the principal plane lies in the anterior chamber. 
 
 

3. The back vertex power:

a. is the reciprocal of the back vertex distance

b. of a convex meniscus lens can be calculated from its second focal length

c. of a convex meniscus lens is stronger than its front vertex power

d. gives the equivalent power of a lens

e. is used to grade spectacle lenses
 

Answera.F   b.F   c.T   d.F   e.T 
The concept of back vertex power is used in the calculation of a thick lens.
The back vertex power is the reciprocal of the posterior vertex focal length expressed in dioptres. The posterior vertex focal length is different from the focal length of a thick lens. In the case of a convex meniscus lens, 
the posterior vertex focal length is shorter than both the second focal length and the anterior vertex focal length. As a result, in a convex meniscus lens, the back vertex power is stronger than its front vertex power. 
The equivalent power of thick lens is calculated from the two surface powers and a correction for vergence change due to lens thickness.
Spectacle glasses are graded according to its vertex power because its back vertex power is the one that is used to correct the ametropia. 
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4. The following definitions are true for accommodation:

a. the far point of distinct vision of an emmetropic eye is at infinity 

b. the near point of distinct vision refers to clear near vision when maximum 
    accommodation is used.

c. range of accommodation is the difference in dioptric power between the 
    eye at rest and the fully accommodated eye

d. dynamic refraction refers to the dioptric power of the accommodated eye

e. static refraction refers to the dioptric power of a resting eye
 

Answera.T   b.T   cf.   d.T  e.T
Range of accommodation refers to the distance between the far point and the near point. The difference in dioptric power between the eye at rest and the fully accommodated eye is called the amplitude of accommodation.
 
 

5. Regarding accommodative convergence / accommodation ratio:

a. the eye could not accommodate in the absence of convergence

b. the normal range of accommodative convergence/ accommodation ratio
    is 3:1 to 5:1

c. the interpupillary distance needs to be known if the ratio is to be 
    calculated using the gradient method

d. the value obtained using the heterophoria method tends to be lower than
    that calculated using the gradient method

e. esotropia that occurs as a result of too high an AC/A ratio tends to have
    a larger angle of deviation for near than for distance.
 

Answera.F   b.T   cf.   d.F   e.T
The eye can accommodate in the absence of convergence. However, in normal daily life a patient with normal binocular vision will convergence 
when he/she accommodates. The normal ratio is 3:1 to 5:1. The AC/A 
ratio can be calculated using either a heterophoria method in which the interpupillary distance needs to be known or the gradient method in which the interpupillary distance needs not be measured. The heterophoria method tends to give a higher ratio than the gradient method. Esotropia due to high AC/A ratio tends to be worse for near than distance and can be 
corrected with bifocal glasses to reduce the accommodation or recession of medial recti.
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a6. The catoptric images :

a. are formed at the refracting interfaces of the eye

b. can be used to measure the ocular accommodation

c. are all virtual images

d. are all erect images

e. are made up of 2 images produced by the cornea and 2 images 
    by the crystalline lens
 

Answera.F   b.T   c.F   d.F   e.T
They are also called the Purkinje-Sanson images. The images are
formed at 4 surfaces: the anterior (image 1) and posterior (image 2) corneal surfaces and the anterior (image 3) and posterior (image 4) 
lenticular surfaces. The first three images are erect and virtual whereas
the last one is inverted and real. The first image is used for keratometry and images 3 and 4 are used for accommodation.

 

7. The first image of the captoptric image can be used for:

a. measuring ocular deviation in strabismic patient

b. keratometry

c. measuring accommodation

d. measuring corneal thickness

e. measuring anterior chamber depth
 

Answera.T   b.T   cf.   d.T   e.F
Hirschberg's test made use of the first captoptric image which is located on the anterior corneal surface for measuring ocular deviation. This image is also used for keratometry. The distance between the first and the second images is used to measure the corneal thickness. The depth of the anterior chamber is between the second and the third image. Accommodation is the thickness between the third and the fourth images.