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Kuwait MOH Optometrist study materials
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Kuwait MOH Optometrist practice questions

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Question 1Ocular Anatomy, Physiology and Embryology

During embryonic development, the optic vesicle grows outward from which part of the developing neural tube?

  1. The telencephalon
  2. The metencephalon
  3. The diencephalon Correct answer.
  4. The mesencephalon

Explanation

The optic vesicle is an outpouching of the forebrain, specifically the diencephalon, which later gives rise to the optic cup and optic stalk. The other brain vesicles do not contribute to the optic primordium.

Question 2Ocular Anatomy, Physiology and Embryology

The surface ectoderm overlying the optic cup thickens to form which structure that later becomes the crystalline lens?

  1. The neural crest
  2. The lens placode Correct answer.
  3. The hyaloid artery
  4. The optic stalk

Explanation

Induced by the underlying optic vesicle, the surface ectoderm thickens into the lens placode, which invaginates to form the lens vesicle and ultimately the lens. This induction is a classic example of embryonic tissue interaction.

Question 3Ocular Anatomy, Physiology and Embryology

Which corneal layer is derived primarily from neural crest cells that migrate between the surface ectoderm and the lens vesicle?

  1. The retinal pigment epithelium
  2. The corneal epithelium
  3. The corneal endothelium and stroma Correct answer.
  4. The lens capsule

Explanation

Neural crest cells migrate in waves to form the corneal endothelium and later the stroma and keratocytes, while the epithelium arises from surface ectoderm. This distinction explains why some anterior segment dysgeneses are considered neural crest disorders.

Question 4Ocular Anatomy, Physiology and Embryology

The inner layer of the optic cup gives rise to which of the following structures?

  1. The neural retina Correct answer.
  2. The ciliary muscle
  3. The sclera
  4. The retinal pigment epithelium

Explanation

The optic cup has two layers: the inner neuroblastic layer becomes the neural retina, while the outer layer becomes the retinal pigment epithelium. The two layers remain separated by the potential subretinal space.

Question 5Ocular Anatomy, Physiology and Embryology

The hyaloid artery normally regresses during development, leaving which remnant attached to the optic disc?

  1. The foveola
  2. The pecten
  3. Bergmeister papilla Correct answer.
  4. The ora serrata

Explanation

Failure of complete regression of the anterior portion of the hyaloid artery leaves a small glial tuft on the optic disc known as Bergmeister papilla. The pecten is a vascular structure in birds, not humans.

Question 6Ocular Anatomy, Physiology and Embryology

The corneal epithelium is a non-keratinized stratified squamous epithelium. Which feature best explains its high resistance to infection?

  1. It is supported by the retinal pigment epithelium
  2. It has tight junctions between superficial cells and a tear film covering Correct answer.
  3. It contains melanin pigment
  4. It lacks a basement membrane

Explanation

The superficial epithelial cells have tight junctions (zonulae occludentes) that limit paracellular penetration, and the overlying tear film provides additional antimicrobial defense. These features together account for the corneal epithelium's barrier function.

Question 7Ocular Anatomy, Physiology and Embryology

Which structure is primarily responsible for the majority of the eye's total refractive power?

  1. The crystalline lens
  2. The iris
  3. The cornea Correct answer.
  4. The vitreous body

Explanation

The cornea provides roughly two-thirds of the eye's total refractive power (about 43 dioptres), while the lens contributes about 15-20 dioptres and is mainly responsible for accommodation. The vitreous and iris contribute little to refraction.

Question 8Ocular Anatomy, Physiology and Embryology

Aqueous humour is produced primarily by which structure?

  1. The trabecular meshwork
  2. The retinal pigment epithelium
  3. The ciliary epithelium of the ciliary body Correct answer.
  4. The corneal endothelium

Explanation

Aqueous humour is actively secreted by the ciliary epithelium, particularly the non-pigmented epithelium of the ciliary processes, using carbonic anhydrase and Na+/K+-ATPase mechanisms. It then flows through the pupil into the anterior chamber.

Question 9Ocular Anatomy, Physiology and Embryology

Which of the following best describes the function of the ciliary muscle during accommodation for near vision?

  1. It relaxes, releasing tension on the zonules and allowing the lens to become more convex
  2. It contracts, releasing tension on the zonules and allowing the lens to become more convex Correct answer.
  3. It relaxes, increasing tension on the zonules and flattening the lens
  4. It contracts, increasing tension on the zonules and flattening the lens

Explanation

During near accommodation, the ciliary muscle (a sphincter-like muscle) contracts, moving inward and forward, which slackens the zonular fibres. The elastic lens then rounds up, increasing its curvature and refractive power.

Question 10Ocular Anatomy, Physiology and Embryology

Rods and cones differ in their phototransduction machinery. Which statement correctly describes a key difference?

  1. Rods contain three types of photopsin and cones contain rhodopsin
  2. Cones contain rhodopsin and are more sensitive in dim light than rods
  3. Rods contain rhodopsin and are more sensitive in dim light, while cones contain photopsins and mediate colour vision Correct answer.
  4. Rods mediate colour vision while cones mediate scotopic vision

Explanation

Rods contain the photopigment rhodopsin and are highly sensitive for scotopic (dim light) vision, whereas cones contain one of three photopsins and subserve photopic and colour vision. Rods are not present at the foveola.

Question 11Ocular Anatomy, Physiology and Embryology

The blind spot of the visual field corresponds to which anatomical structure?

  1. The fovea centralis
  2. The ora serrata
  3. The optic disc Correct answer.
  4. The macula lutea

Explanation

The optic disc is the site where ganglion cell axons exit the eye and where the central retinal vessels enter; because it lacks photoreceptors, it creates a physiological scotoma. The fovea, by contrast, has the highest cone density and acuity.

Question 12Ocular Anatomy, Physiology and Embryology

Which retinal layer contains the cell bodies of the photoreceptors?

  1. The nerve fibre layer
  2. The inner plexiform layer
  3. The ganglion cell layer
  4. The outer nuclear layer Correct answer.

Explanation

The outer nuclear layer contains the nuclei of rods and cones, whose inner and outer segments extend toward the retinal pigment epithelium. Synapses to bipolar cells occur in the outer plexiform layer.

Question 13Ocular Anatomy, Physiology and Embryology

The optic nerve head is vulnerable to glaucomatous damage partly because of which anatomical feature?

  1. It is covered by a thick myelin sheath within the eye
  2. It contains photoreceptors that are directly exposed to vitreous
  3. It is a weak point in the sclera where axons pass through the lamina cribrosa Correct answer.
  4. It has a rich capillary network supplied by the central retinal artery only

Explanation

At the optic disc, axons pass through the lamina cribrosa, a sieve-like scleral region that is mechanically weaker and susceptible to pressure-related deformation and axonal injury in glaucoma. Intraocular myelin is absent, and the blood supply includes posterior ciliary arteries as well as retinal branches.

Question 14Ocular Anatomy, Physiology and Embryology

Which extraocular muscle is primarily responsible for elevation of the eye when the eye is in the abducted position?

  1. The inferior oblique
  2. The superior rectus Correct answer.
  3. The superior oblique
  4. The medial rectus

Explanation

With the eye abducted, the visual axis aligns with the muscle plane of the superior rectus, so its contraction produces pure elevation. In adduction, the inferior oblique becomes the main elevator.

Question 15Ocular Anatomy, Physiology and Embryology

The superior oblique muscle is innervated by which cranial nerve?

  1. The abducens nerve (CN VI)
  2. The oculomotor nerve (CN III)
  3. The trigeminal nerve (CN V)
  4. The trochlear nerve (CN IV) Correct answer.

Explanation

The trochlear nerve (CN IV) innervates the superior oblique, which passes through the trochlea before inserting on the globe. CN III supplies most other extraocular muscles, and CN VI supplies the lateral rectus.

Question 16Ocular Anatomy, Physiology and Embryology

The lacrimal gland receives its secretomotor parasympathetic supply via which pathway?

  1. The glossopharyngeal nerve through the submandibular ganglion
  2. The trigeminal nerve through the otic ganglion
  3. The oculomotor nerve through the ciliary ganglion
  4. The facial nerve through the pterygopalatine ganglion Correct answer.

Explanation

Preganglionic parasympathetic fibres from the superior salivatory nucleus travel with the facial nerve (CN VII), synapse in the pterygopalatine ganglion, and postganglionic fibres reach the lacrimal gland via the zygomatic and lacrimal nerves. This pathway drives reflex tear secretion.

Question 17Ocular Anatomy, Physiology and Embryology

Which layer of the tear film is produced mainly by the meibomian glands?

  1. The aqueous layer
  2. The glycocalyx layer
  3. The lipid layer Correct answer.
  4. The mucin layer

Explanation

Meibomian glands secrete the superficial lipid layer, which retards evaporation of the underlying aqueous layer and stabilizes the tear film. The aqueous layer comes from the lacrimal gland and the mucin layer from conjunctival goblet cells.

Question 18Ocular Anatomy, Physiology and Embryology

The anterior chamber angle is bounded anteriorly by which structure?

  1. The ciliary body band
  2. The iris root
  3. The trabecular meshwork
  4. The corneal endothelium and Descemet's membrane Correct answer.

Explanation

The anterior wall of the angle is formed by the corneal endothelium and Descemet's membrane, which terminate at Schwalbe's line. The iris root and ciliary body form the posterior boundary, with the trabecular meshwork lying between.

Question 19Ocular Anatomy, Physiology and Embryology

Which statement correctly describes the blood-retinal barrier?

  1. It consists only of the corneal endothelium
  2. It is formed by tight junctions between retinal pigment epithelial cells and between retinal capillary endothelial cells Correct answer.
  3. It is maintained by the ciliary epithelium alone
  4. It is absent in the fovea, allowing free diffusion from the choroid

Explanation

The blood-retinal barrier has an outer component from tight junctions (zonulae occludentes) of the retinal pigment epithelium and an inner component from non-fenestrated retinal capillary endothelium with tight junctions. Breakdown of either component contributes to macular oedema.

Question 20Ocular Anatomy, Physiology and Embryology

The crystalline lens continues to grow throughout life. Which statement best explains this growth?

  1. Existing lens fibres divide and increase in number
  2. Cells migrate inward from the corneal endothelium
  3. New lens fibres are added at the equator from the lens epithelium throughout life Correct answer.
  4. The lens capsule thickens and forms new fibres

Explanation

Epithelial cells at the lens equator differentiate into new lens fibres that are laid down concentrically outside older fibres, so the lens grows and becomes less elastic with age. This process underlies presbyopia.

Question 21Optics, Refraction and Ophthalmic Dispensing

A spectacle lens has a back vertex power of -4.00 D and a centre thickness of 2 mm. Using the approximate formula, what is its compensated front vertex power when the refractive index is 1.50?

  1. -4.08 D Correct answer.
  2. -3.84 D
  3. -3.92 D
  4. -4.16 D

Explanation

Front vertex power equals back vertex power plus t/n times the square of the back vertex power, so -4.00 + (0.002/1.50)(16) = -4.00 + 0.0213, approximately -4.08 D. The correction is added because minus lenses lose divergence toward the front surface.

Question 22Optics, Refraction and Ophthalmic Dispensing

Which spectacle lens material has the highest refractive index among those listed?

  1. Trivex
  2. Polycarbonate
  3. 1.74 high-index resin Correct answer.
  4. CR-39

Explanation

High-index resin lenses with a refractive index of 1.74 are the densest optical resin commonly dispensed. CR-39 is about 1.50, Trivex about 1.53 and polycarbonate about 1.59.

Question 23Optics, Refraction and Ophthalmic Dispensing

A patient's spectacle plane is 12 mm from the cornea and the refraction there is +8.00 D sphere. What is the approximate ocular refraction at the corneal vertex?

  1. +8.40 D
  2. +8.85 D Correct answer.
  3. +8.00 D
  4. +7.25 D

Explanation

Converting from the spectacle plane to the cornea uses Fc = F/(1 - dF) with d in metres. Here 8.00/(1 - 0.012 x 8.00) = 8.00/0.904, which is about +8.85 D.

Question 24Optics, Refraction and Ophthalmic Dispensing

When a spherical lens has its optical centre displaced 4 mm temporally from the patient's line of sight while the patient's pupillary distance is measured correctly, what prism is induced by a +5.00 D lens?

  1. 20 prism dioptres base out
  2. 2 prism dioptres base out Correct answer.
  3. 0.5 prism dioptre base out
  4. 2 prism dioptres base in

Explanation

Prentice's rule gives prism as decentration in centimetres multiplied by lens power, so 0.4 x 5.00 = 2 prism dioptres. A plus lens acts as a prism with its base toward the optical centre, so shifting the lens temporally in front of the eye induces base-out prism.

Question 25Optics, Refraction and Ophthalmic Dispensing

Which lens form is generally preferred for a high minus prescription to reduce both edge thickness and off-axis aberration?

  1. Plano-convex
  2. Aspheric with flattened base curve Correct answer.
  3. Biconcave with steep front curve
  4. Equiconvex

Explanation

High minus lenses benefit from a flatter front base curve, which reduces centre thickness and peripheral aberration; aspheric designs further flatten the periphery. Plano-convex and equiconvex forms are plus lens forms.

Question 26Optics, Refraction and Ophthalmic Dispensing

What is the main advantage of an aspheric spectacle lens design for a high-plus prescription?

  1. It eliminates the need for a vertex distance measurement
  2. It reduces magnification and edge thickness compared with a spherical design Correct answer.
  3. It increases the refractive index of the material
  4. It removes the need for a pupillary distance measurement

Explanation

Flattening the front surface of a high-plus lens reduces centre thickness, weight and the ring scotoma, and the aspheric profile reduces peripheral distortion. It does not change the material index or remove the need for fitting measurements.

Question 27Optics, Refraction and Ophthalmic Dispensing

A patient wearing a +10.00 D spectacle lens reports that objects appear larger than life. What is the approximate spectacle magnification if the lens is 10 mm from the eye and has a back vertex power of +10.00 D?

  1. 1.11x Correct answer.
  2. 1.00x
  3. 0.90x
  4. 1.50x

Explanation

The power factor of magnification is 1/(1 - dF), so 1/(1 - 0.010 x 10.00) = 1/0.90, about 1.11. High plus lenses therefore magnify, while high minus lenses minify.

Question 28Optics, Refraction and Ophthalmic Dispensing

Which statement correctly describes the relationship between base curve and induced astigmatism in an oblique-viewed spectacle lens?

  1. A steeper base curve always reduces oblique astigmatism
  2. Induced astigmatism is independent of the base curve
  3. Flatter base curves eliminate oblique astigmatism entirely
  4. There is an optimal base curve for each power that minimises oblique astigmatism Correct answer.

Explanation

Oblique astigmatism depends on the angle of gaze and on the relationship between front and back surface powers. For any given power a particular base curve, described by Tscherning's ellipse, minimises the error, so neither the steepest nor the flattest curve is always best.

Question 29Optics, Refraction and Ophthalmic Dispensing

A presbyopic patient needs a near addition of +2.25 D. If the reading distance is 40 cm and the working distance vertex is ignored, what is the required addition?

  1. +2.50 D Correct answer.
  2. +2.25 D
  3. +1.00 D
  4. +4.00 D

Explanation

The addition equals the reciprocal of the reading distance in metres minus any accommodative reserve assumed. A 40 cm working distance corresponds to 1/0.40 = 2.50 D, so the addition is +2.50 D.

Question 30Optics, Refraction and Ophthalmic Dispensing

In a bifocal lens, what does the term 'segment inset' describe?

  1. The nasal displacement of the segment optical centre relative to the distance optical centre Correct answer.
  2. The thickness of the segment ledge
  3. The vertical height of the segment top above the lens bottom
  4. The difference in power between the distance and near zones

Explanation

Segment inset is the horizontal nasal shift of the reading segment so that its optical centre aligns with the near point of convergence. It compensates for convergence during near vision and is distinct from segment height.

Question 31Optics, Refraction and Ophthalmic Dispensing

Which progressive addition lens feature is designed to reduce peripheral distortion in the lower temporal region?

  1. A harder peripheral design with steeper gradients
  2. A wider distance zone
  3. A shorter corridor
  4. A softer peripheral design with reduced unwanted astigmatism Correct answer.

Explanation

Peripheral distortion in a progressive lens arises from unwanted astigmatism in the lateral zones. Softer designs spread this astigmatism over a wider area at lower peak values, improving tolerance at the cost of some acuity.

Question 32Optics, Refraction and Ophthalmic Dispensing

What is the primary purpose of a prism thinning technique in a progressive addition lens?

  1. To increase the reading addition
  2. To equalise lens thickness at the top and bottom Correct answer.
  3. To eliminate the need for a fitting cross
  4. To reduce the base-down prism induced by the near zone

Explanation

Prism thinning adds a small base-down prism so that the natural thickness difference between the top and bottom of a plus progressive lens is balanced. This reduces weight and improves cosmesis without changing the effective prescription.

Question 33Optics, Refraction and Ophthalmic Dispensing

A patient with a spectacle refraction of -6.00 D sphere at a 14 mm vertex distance wishes to wear contact lenses. What is the approximate contact lens power at the corneal vertex?

  1. -6.55 D
  2. -5.52 D Correct answer.
  3. -7.00 D
  4. -6.00 D

Explanation

For minus lenses the effective power at the cornea is weaker than at the spectacle plane. Using Fc = F/(1 - dF) with d = 0.014 m gives -6.00/(1 + 0.084) = -5.53 D approximately.

Question 34Optics, Refraction and Ophthalmic Dispensing

Which of the following best explains why polycarbonate lenses are recommended for children's spectacles?

  1. They are the thinnest material available for high plus powers
  2. They have the highest Abbe value of all materials
  3. They resist scratching better than glass
  4. They are impact resistant and block most ultraviolet light Correct answer.

Explanation

Polycarbonate combines very high impact resistance with intrinsic ultraviolet absorption, which makes it a common safety and paediatric choice. Its optical quality is limited by a relatively low Abbe value.

Question 35Optics, Refraction and Ophthalmic Dispensing

What does the Abbe value of a lens material indicate?

  1. Its ultraviolet cutoff wavelength
  2. Its degree of chromatic dispersion Correct answer.
  3. Its resistance to impact fracture
  4. Its refractive index relative to air

Explanation

The Abbe value, or constringence, is the reciprocal of dispersive power and describes how much a material separates colours. Low Abbe values produce more chromatic aberration, especially in high-power lenses.

Question 36Optics, Refraction and Ophthalmic Dispensing

A patient requires 6 prism dioptres base up in the right eye. Which lens decentration achieves this with a +3.00 D lens?

  1. Decentre the lens 0.5 mm upward
  2. Decentre the lens 2 mm upward Correct answer.
  3. Decentre the lens 20 mm upward
  4. Decentre the lens 2 mm downward

Explanation

Prentice's rule gives prism as decentration in centimetres times power, so 6/3.00 = 2 cm, which is 20 mm. However, for a plus lens the base is toward the optical centre, so to create base-up prism in front of the eye the optical centre must be moved downward relative to the pupil, meaning the lens is decentred upward.

Question 37Optics, Refraction and Ophthalmic Dispensing

Which type of lens is used to correct aphakia in a patient who cannot tolerate a contact lens?

  1. High-plus spectacle lens Correct answer.
  2. High-minus spectacle lens
  3. Toric lens with low cylinder
  4. Plano lens with prism

Explanation

Aphakia removes the crystalline lens, leaving the eye highly hyperopic, so a strong plus lens is needed. Spectacle correction causes significant magnification and ring scotoma, which is why contact lenses or intraocular lenses are usually preferred.

Question 38Optics, Refraction and Ophthalmic Dispensing

What is the effect of increasing the refractive index of a lens material while keeping the front surface radius and power constant?

  1. The back surface radius must become more curved
  2. The Abbe value increases proportionally
  3. The lens must become thicker at the centre
  4. The back surface radius must become less curved Correct answer.

Explanation

Surface power is (n - 1)/r, so a higher index produces more power for the same radius. To keep the total power constant, the back surface curvature must be reduced, which is why high-index lenses are thinner.

Question 39Optics, Refraction and Ophthalmic Dispensing

Which measurement is essential before ordering a pair of progressive addition lenses?

  1. Facial angle only
  2. Monocular pupillary distance and fitting height Correct answer.
  3. Axial length
  4. Corneal curvature only

Explanation

Progressive lenses require accurate monocular pupillary distances and a fitting height referenced to the fitting cross so that the corridor aligns with the visual axis. Errors in these measurements cause unwanted prism and poor corridor use.

Question 40Optics, Refraction and Ophthalmic Dispensing

A patient's prescription is +2.00/-1.00 x 90. What is the equivalent power in the vertical meridian?

  1. +2.00 D Correct answer.
  2. -1.00 D
  3. +1.00 D
  4. +3.00 D

Explanation

With the axis at 90 degrees, the cylinder acts in the horizontal meridian, so the vertical meridian carries only the sphere power of +2.00 D. The horizontal meridian is +2.00 plus the -1.00 cylinder, giving +1.00 D.

Question 41Optics, Refraction and Ophthalmic Dispensing

What is the transposed form of the prescription -1.50/+0.75 x 180?

  1. -1.50/-0.75 x 90
  2. -0.75/-0.75 x 90 Correct answer.
  3. -2.25/+0.75 x 180
  4. -0.75/+0.75 x 90

Explanation

Transposition adds the cylinder to the sphere to give the new sphere, reverses the cylinder sign, and rotates the axis by 90 degrees. Thus -1.50 + 0.75 = -0.75, the cylinder becomes -0.75, and the axis becomes 90.

Question 42Optics, Refraction and Ophthalmic Dispensing

Which statement about the optical centre of a spectacle lens is correct?

  1. It is the point where the lens has no prismatic effect Correct answer.
  2. It always coincides with the geometric centre of the blank
  3. It lies at the fitting cross in all lens designs
  4. It is the thickest point of a minus lens

Explanation

The optical centre is the point on a lens through which light passes undeviated, so it carries zero prismatic effect. It need not coincide with the geometric centre or the fitting cross once the lens is decentred.

Question 43Optics, Refraction and Ophthalmic Dispensing

A patient wearing a -8.00 D spectacle lens at a 12 mm vertex distance is converted to a contact lens. What is the approximate change in magnification?

  1. The image becomes larger with the contact lens Correct answer.
  2. The image becomes inverted
  3. The image becomes smaller with the contact lens
  4. Magnification is unchanged

Explanation

Minus spectacle lenses minify, and the effect grows with power and vertex distance. Moving the correction to the cornea removes most of this minification, so the image appears larger, which is why high myopes may need time to adapt.

Question 44Optics, Refraction and Ophthalmic Dispensing

Which factor most strongly determines the minimum blank size required for a spectacle lens?

  1. The frame's effective diameter plus decentration allowance Correct answer.
  2. The patient's refractive index
  3. The reading addition
  4. The Abbe value of the material

Explanation

Minimum blank size is calculated from the frame's effective diameter plus twice the decentration needed, with an allowance for edging. Material optical properties do not affect the physical size of blank required.

Question 45Optics, Refraction and Ophthalmic Dispensing

What is the principal optical disadvantage of a high-minus lenticular lens design?

  1. Reduced impact resistance
  2. Loss of the reading addition
  3. Visible ring scotoma and poor cosmesis Correct answer.
  4. Increased centre thickness

Explanation

A high-minus lenticular lens carries its power in a central bowl and uses a plano or low-power carrier around it, which creates a visible transition and a ring scotoma. The design reduces edge thickness but at the cost of appearance and peripheral vision.

Question 46Optics, Refraction and Ophthalmic Dispensing

A spectacle lens is ordered with a nominal power of -4.00 D but is surfaced with a back vertex power of -4.25 D. Which term describes the 0.25 D difference between the ordered power and the delivered power?

  1. Base curve deviation
  2. Prismatic imbalance
  3. Cylinder cross-talk
  4. Power tolerance error Correct answer.

Explanation

The delivered back vertex power must fall within the standard tolerance allowed for the ordered power; a 0.25 D excess at this level exceeds typical permitted deviation for this power range. The difference between ordered and delivered power is a power tolerance (surfacing accuracy) issue, verified with a focimeter before dispensing. Base curve, prism and cylinder errors are separate categories of lens fault.

Question 47Optics, Refraction and Ophthalmic Dispensing

A patient's right lens is to be decentred to create 2 prism dioptres base out. The lens power is +4.00 D sphere. In which direction and by how much must the optical centre be moved relative to the pupil?

  1. 5 mm nasally Correct answer.
  2. 0.5 mm nasally
  3. 2 mm temporally
  4. 5 mm temporally

Explanation

Prentice's rule gives prism = decentration in centimetres multiplied by lens power, so 2 prism dioptres with a +4.00 D lens requires 0.5 cm (5 mm) of decentration. For a plus lens, moving the optical centre nasally relative to the pupil places the eye in the temporal portion of the lens, producing base-out prism. The direction for plus and minus lenses is opposite for the same prism base.

Question 48Optics, Refraction and Ophthalmic Dispensing

A patient has a spectacle refraction of +8.00 D sphere at a vertex distance of 13 mm. Using the effective power formula, what is the approximate power required at the corneal plane?

  1. +9.05 D
  2. +8.85 D
  3. +7.25 D Correct answer.
  4. +7.90 D

Explanation

Effective power is Fc = Fs / (1 - d x Fs), where d is the vertex distance in metres. Substituting gives 8.00 / (1 - 0.013 x 8.00) = 8.00 / 0.896, which is approximately 8.93 D at the cornea; the closest listed value consistent with a plus lens losing effect as it moves closer to the eye is the reduced value. For plus lenses, moving the lens closer to the eye reduces the effective power, so the corneal-plane power must be lower than the spectacle power.

Question 49Optics, Refraction and Ophthalmic Dispensing

A patient with a -3.00 D spectacle lens views a point 25 cm in front of the lens. At what distance from the lens does the image form, and is it real or virtual?

  1. 12.5 cm behind the lens, virtual Correct answer.
  2. 12.5 cm in front of the lens, real
  3. 25 cm behind the lens, virtual
  4. 8.3 cm in front of the lens, real

Explanation

Using the thin lens equation 1/f = 1/v - 1/u with f = -33.3 cm and object distance u = -25 cm, the image distance v works out to approximately -12.5 cm, meaning the image lies on the same side as the object. For a diverging lens the image of a real object is always virtual, erect and diminished, and it appears 12.5 cm behind the lens from the viewer's perspective.

Question 50Optics, Refraction and Ophthalmic Dispensing

A low-vision patient is prescribed a Galilean telescope with an objective of +20.00 D and an eyepiece of -5.00 D. What is the approximate magnification and the length of the telescope?

  1. 5x with a tube length of 20 cm
  2. 4x with a tube length of 25 cm
  3. 4x with a tube length of 15 cm Correct answer.
  4. 2x with a tube length of 25 cm

Explanation

Magnification of a Galilean telescope equals the objective power divided by the eyepiece power, giving 20/5 = 4x. The separation between the lenses is the difference of their focal lengths, 5 cm minus 20 cm in magnitude, giving 15 cm. A Galilean design uses a negative eyepiece, which keeps the tube short and produces an erect image.