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Arab Board Ophthalmology Part 1 study materials
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Arab Board Ophthalmology Part 1 practice questions

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Question 1Basic Sciences: Anatomy, Physiology and Embryology of the Eye

During development, the neurosensory retina is derived from which embryologic tissue?

  1. Surface ectoderm
  2. Endoderm of the foregut
  3. Neural ectoderm of the optic cup Correct answer.
  4. Mesenchyme of neural crest origin

Explanation

The optic vesicle evaginates from the forebrain (neural ectoderm) and invaginates to form the bilayered optic cup; the inner layer becomes the neurosensory retina. Surface ectoderm contributes the lens, and neural crest mesenchyme forms the sclera and corneal stroma.

Question 2Basic Sciences: Anatomy, Physiology and Embryology of the Eye

The corneal endothelium and the lens epithelium share which embryologic origin?

  1. Neural ectoderm
  2. Mesoderm
  3. Neural crest cells
  4. Surface ectoderm Correct answer.

Explanation

The lens arises from the surface ectoderm-derived lens placode, and its anterior layer persists as the lens epithelium. The corneal endothelium is also derived from surface ectoderm after the lens vesicle separates.

Question 3Basic Sciences: Anatomy, Physiology and Embryology of the Eye

The hyaloid artery in the fetus normally regresses to become which adult structure?

  1. Cloquet's canal Correct answer.
  2. Central retinal artery
  3. Circle of Zinn
  4. Long posterior ciliary artery

Explanation

The hyaloid artery runs through the primary vitreous and normally regresses, leaving the avascular central vitreous channel known as Cloquet's canal. Persistence results in a Mittendorf dot or Bergmeister papilla.

Question 4Basic Sciences: Anatomy, Physiology and Embryology of the Eye

Which structure forms the boundary between the corneal stroma and the corneal endothelium?

  1. Descemet's membrane Correct answer.
  2. Bowman's layer
  3. Epithelial basement membrane
  4. Dua's layer

Explanation

Descemet's membrane is the thick basement membrane secreted by the corneal endothelium and lies directly between the stroma and endothelium. It is resistant to collagenases and thickens with age.

Question 5Basic Sciences: Anatomy, Physiology and Embryology of the Eye

The corneal epithelium is maintained by a stem cell population located in which region?

  1. Palpebral conjunctiva
  2. Central corneal epithelium
  3. Limbus Correct answer.
  4. Conjunctival fornix

Explanation

Limbal basal epithelial stem cells reside in the palisades of Vogt at the corneoscleral junction and continuously repopulate the corneal epithelium. Loss of these cells leads to conjunctivalization of the cornea.

Question 6Basic Sciences: Anatomy, Physiology and Embryology of the Eye

Aqueous humor drains from the anterior chamber into the episcleral veins primarily via which pathway?

  1. Schlemm's canal to the vitreous
  2. Iris root to ciliary processes
  3. Ciliary body to suprachoroidal space
  4. Trabecular meshwork to Schlemm's canal Correct answer.

Explanation

The conventional (pressure-dependent) outflow route passes through the trabecular meshwork into Schlemm's canal and then via collector channels to the episcleral veins. This pathway accounts for the majority of aqueous outflow in humans.

Question 7Basic Sciences: Anatomy, Physiology and Embryology of the Eye

Aqueous humor is actively secreted primarily by which structure?

  1. Ciliary epithelium Correct answer.
  2. Trabecular meshwork
  3. Iris pigment epithelium
  4. Corneal endothelium

Explanation

The bilayered ciliary epithelium, especially the non-pigmented layer, actively secretes aqueous humor using carbonic anhydrase and Na+/K+-ATPase. This active secretion is the target of carbonic anhydrase inhibitors.

Question 8Basic Sciences: Anatomy, Physiology and Embryology of the Eye

Which cells are responsible for producing the myelin sheath of the optic nerve?

  1. Astrocytes
  2. Oligodendrocytes Correct answer.
  3. Schwann cells
  4. Muller cells

Explanation

The optic nerve is a central nervous system tract, so its axons are myelinated by oligodendrocytes rather than Schwann cells. This explains why optic neuritis occurs in demyelinating disease such as multiple sclerosis.

Question 9Basic Sciences: Anatomy, Physiology and Embryology of the Eye

The optic nerve is surrounded by meninges; the subarachnoid space around the intraorbital optic nerve is continuous with which space?

  1. Vitreous cavity
  2. Suprachoroidal space
  3. Subarachnoid space of the brain Correct answer.
  4. Subdural space of the brain

Explanation

The optic nerve sheath is an extension of the cerebral meninges, and the subarachnoid space around the nerve communicates with the intracranial subarachnoid space. This is why raised intracranial pressure produces papilledema.

Question 10Basic Sciences: Anatomy, Physiology and Embryology of the Eye

The blood supply to the inner retina is derived mainly from which vessel?

  1. Anterior ciliary arteries
  2. Long posterior ciliary arteries
  3. Choroidal capillary bed
  4. Central retinal artery Correct answer.

Explanation

The central retinal artery supplies the inner retinal layers, while the outer retina is supplied by the choroid. This dual supply makes the inner retina vulnerable to central retinal artery occlusion.

Question 11Basic Sciences: Anatomy, Physiology and Embryology of the Eye

The fovea centralis is specialized for high-acuity vision because it has which feature?

  1. High density of retinal vessels
  2. High density of rods and thick ganglion layer
  3. Absence of inner retinal layers over a cone-rich region Correct answer.
  4. Large number of bipolar cells per cone

Explanation

At the foveola, inner retinal layers are displaced laterally (foveal pit), allowing light to reach densely packed cones directly. This arrangement maximizes visual acuity.

Question 12Basic Sciences: Anatomy, Physiology and Embryology of the Eye

Which retinal cell type forms the first synapse in the vertical visual pathway?

  1. Amacrine cells
  2. Bipolar cells Correct answer.
  3. Horizontal cells
  4. Ganglion cells

Explanation

Photoreceptors synapse onto bipolar cells, which then synapse onto ganglion cells. This makes bipolar cells the second-order neurons of the vertical pathway.

Question 13Basic Sciences: Anatomy, Physiology and Embryology of the Eye

Rods are more sensitive to dim light than cones primarily because rods have which property?

  1. Faster phototransduction kinetics
  2. Higher concentration of iodopsin
  3. Greater convergence onto bipolar cells Correct answer.
  4. Shorter outer segments

Explanation

Many rods converge onto a single bipolar and ganglion cell, producing spatial summation that increases light sensitivity. This convergence sacrifices acuity, which is why rods subserve scotopic vision.

Question 14Basic Sciences: Anatomy, Physiology and Embryology of the Eye

Vitamin A deficiency causes night blindness because which molecule requires retinaldehyde as a cofactor?

  1. Myelin basic protein
  2. Melanin
  3. Carbonic anhydrase
  4. Rhodopsin Correct answer.

Explanation

11-cis-retinal, derived from vitamin A, is the chromophore of rhodopsin in rod photoreceptors. Deficiency impairs rhodopsin regeneration and causes impaired dark adaptation.

Question 15Basic Sciences: Anatomy, Physiology and Embryology of the Eye

When the eye accommodates for near vision, which change occurs?

  1. Ciliary muscle contracts and lens thickens Correct answer.
  2. Iris dilates and lens thickens
  3. Zonules tighten and lens flattens
  4. Ciliary muscle relaxes and lens flattens

Explanation

Accommodation involves contraction of the ciliary muscle, which reduces zonular tension and allows the elastic lens to become more convex. This increases refractive power for near objects.

Question 16Basic Sciences: Anatomy, Physiology and Embryology of the Eye

Which extraocular muscle is the primary depressor of the eye in the adducted position?

  1. Superior oblique Correct answer.
  2. Inferior oblique
  3. Lateral rectus
  4. Inferior rectus

Explanation

The superior oblique has an oblique line of action after passing through the trochlea, making it primarily an intorter and depressor, especially when the eye is adducted. Its depressor action is maximal in adduction.

Question 17Basic Sciences: Anatomy, Physiology and Embryology of the Eye

The muscle that elevates the upper eyelid and is innervated by the oculomotor nerve is which of the following?

  1. Superior tarsal muscle
  2. Orbicularis oculi
  3. Levator palpebrae superioris Correct answer.
  4. Frontalis

Explanation

The levator palpebrae superioris is innervated by the superior division of the oculomotor nerve (CN III). It is the principal elevator of the upper lid.

Question 18Basic Sciences: Anatomy, Physiology and Embryology of the Eye

Horner syndrome produces ptosis through loss of innervation to which muscle?

  1. Levator palpebrae superioris
  2. Orbicularis oculi
  3. Superior tarsal muscle Correct answer.
  4. Inferior rectus

Explanation

The superior tarsal (Muller) muscle receives sympathetic innervation and contributes a few millimeters of lid elevation. Sympathetic denervation causes mild ptosis, miosis, and anhidrosis.

Question 19Basic Sciences: Anatomy, Physiology and Embryology of the Eye

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

  1. Greater petrosal nerve branch of the facial nerve Correct answer.
  2. Sympathetic plexus around the internal carotid
  3. Optic nerve
  4. Ophthalmic division of the trigeminal nerve

Explanation

Preganglionic parasympathetic fibers travel with the greater petrosal nerve (from the facial nerve) to the pterygopalatine ganglion, then via the zygomatic and lacrimal nerves to the gland. This drives reflex tearing.

Question 20Basic Sciences: Anatomy, Physiology and Embryology of the Eye

Aqueous humor leaves the anterior chamber and passes through the pupil before reaching which structure first?

  1. Ciliary body
  2. Schlemm's canal
  3. Trabecular meshwork Correct answer.
  4. Suprachoroidal space

Explanation

Aqueous flows from the posterior to anterior chamber through the pupil, then into the trabecular meshwork before reaching Schlemm's canal. The trabecular meshwork provides the main resistance to outflow.

Question 21Basic Sciences: Anatomy, Physiology and Embryology of the Eye

Which of the following best describes the blood-retinal barrier at the level of the retinal pigment epithelium?

  1. Fenestrated capillaries in the choroid with open RPE junctions
  2. Tight junctions between photoreceptor inner segments
  3. Absence of any barrier at the RPE
  4. Tight junctions between RPE cells Correct answer.

Explanation

The outer blood-retinal barrier is formed by tight junctions (zonulae occludentes) between retinal pigment epithelial cells. The inner barrier is formed by tight junctions between retinal capillary endothelial cells.

Question 22Basic Sciences: Anatomy, Physiology and Embryology of the Eye

The superior orbital fissure transmits all of the following except which structure?

  1. Trochlear nerve
  2. Oculomotor nerve
  3. Optic nerve Correct answer.
  4. Abducens nerve

Explanation

The optic nerve passes through the optic canal, not the superior orbital fissure. The oculomotor, trochlear, abducens, and ophthalmic division of the trigeminal nerves pass through the superior orbital fissure.

Question 23Basic Sciences: Anatomy, Physiology and Embryology of the Eye

An infant presents with a white pupillary reflex and is found to have a tumor arising from primitive retinal cells. Which cell type is implicated?

  1. Ciliary epithelial cells
  2. Retinal pigment epithelium
  3. Retinoblastoma cells of neuroblastic origin Correct answer.
  4. Choroidal melanocytes

Explanation

Retinoblastoma arises from primitive neuroectodermal cells of the developing retina, typically from a mutation in both RB1 alleles. It presents in young children with leukocoria.

Question 24Basic Sciences: Anatomy, Physiology and Embryology of the Eye

The lens obtains its nutrients primarily from which source?

  1. Choroidal circulation
  2. Direct arterial branches entering the lens capsule
  3. Aqueous humor and vitreous humor Correct answer.
  4. Retinal circulation

Explanation

The lens is avascular and receives nutrients and oxygen by diffusion from the aqueous humor anteriorly and the vitreous humor posteriorly. This avascularity is important for its transparency.

Question 25Basic Sciences: Anatomy, Physiology and Embryology of the Eye

During embryogenesis, the optic fissure normally closes to allow which structure to form properly?

  1. The lens vesicle
  2. The corneal endothelium
  3. The vitreous body
  4. The choroid fissure and optic nerve head Correct answer.

Explanation

The optic (choroid) fissure closes around the seventh week of gestation, completing the globe and allowing the hyaloid vessels to enter and the optic nerve to form. Failure of closure produces a coloboma.

Question 26Basic Sciences: Anatomy, Physiology and Embryology of the Eye

Which structure forms the boundary between the anterior chamber and the posterior chamber of the eye?

  1. The lens and zonules
  2. The iris and pupil Correct answer.
  3. The cornea and trabecular meshwork
  4. The ciliary body and vitreous

Explanation

The anterior chamber is bounded anteriorly by the cornea and posteriorly by the iris and pupil. The posterior chamber is the space between the iris and the lens; thus the iris, with its central pupil, separates the two chambers. Aqueous humor flows from the posterior chamber through the pupil into the anterior chamber.

Question 27Basic Sciences: Anatomy, Physiology and Embryology of the Eye

During accommodation for near vision, the lens increases its dioptric power primarily by which change?

  1. Increased curvature of the anterior lens surface Correct answer.
  2. Flattening of the posterior lens surface
  3. Relaxation of the ciliary muscle
  4. Increased thickness of the lens capsule

Explanation

Accommodation involves contraction of the ciliary muscle, which relaxes the zonular tension and allows the elastic lens to assume a more spherical shape. The anterior lens surface becomes more convex, increasing its refractive power. The posterior surface changes little compared with the anterior surface.

Question 28Basic Sciences: Anatomy, Physiology and Embryology of the Eye

Which of the following best describes the origin of the corneal epithelium during embryonic development?

  1. Mesoderm
  2. Surface ectoderm Correct answer.
  3. Endoderm
  4. Neural crest cells

Explanation

The corneal epithelium develops from the surface ectoderm that covers the optic cup after lens detachment. The corneal stroma and endothelium arise largely from neural crest cells. This distinction is important because disorders of neural crest migration can affect the stroma and endothelium while sparing the epithelium.

Question 29Basic Sciences: Anatomy, Physiology and Embryology of the Eye

A patient has a lesion causing a left-sided Homer syndrome. Which set of findings is most consistent with this diagnosis?

  1. Left ptosis, left miosis, and left facial anhidrosis Correct answer.
  2. Left ptosis, left miosis, and left facial hyperhidrosis
  3. Left ptosis, left mydriasis, and left facial flushing
  4. Right ptosis, right miosis, and right facial anhidrosis

Explanation

Horner syndrome results from interruption of the sympathetic pathway to the eye and face. It produces ipsilateral ptosis (due to Müller muscle weakness), miosis (due to dilator pupillae weakness), and anhidrosis (due to loss of sudomotor fibers). The findings are on the same side as the lesion.

Question 30Basic Sciences: Anatomy, Physiology and Embryology of the Eye

A 60-year-old man reports a sudden painless loss of vision in one eye. Fundoscopy shows a cherry-red spot at the macula. Which vascular event most likely explains this finding?

  1. Vitreous hemorrhage
  2. Central retinal vein occlusion
  3. Anterior ischemic optic neuropathy
  4. Central retinal artery occlusion Correct answer.

Explanation

Central retinal artery occlusion causes diffuse retinal ischemia and whitening, but the fovea remains red because its blood supply is derived from the choroid, not the retinal circulation. This creates the classic cherry-red spot. The sudden painless loss of vision is typical.

Question 31Ocular Pharmacology and Therapeutics

Which topical agent reduces aqueous humour production by binding to beta-adrenergic receptors on the ciliary epithelium?

  1. Pilocarpine
  2. Brimonidine
  3. Latanoprost
  4. Timolol Correct answer.

Explanation

Beta-blockers such as timolol lower intraocular pressure primarily by decreasing aqueous humour formation at the ciliary epithelium. They do not affect outflow facility or pupil size.

Question 32Ocular Pharmacology and Therapeutics

A patient started on topical dorzolamide reports tingling of the fingers and toes. Which property of this drug best explains this effect?

  1. It is a carbonic anhydrase inhibitor that can cause paraesthesia Correct answer.
  2. It is an osmotic agent that causes fluid shifts
  3. It is a prostaglandin analogue that causes hypertrichosis
  4. It is a cholinesterase inhibitor that causes muscle fasciculation

Explanation

Dorzolamide inhibits carbonic anhydrase, and systemic absorption can produce paraesthesia, often described as tingling of extremities. This is a recognised class effect of carbonic anhydrase inhibitors.

Question 33Ocular Pharmacology and Therapeutics

Which structure must a topically applied drug cross to reach the anterior chamber from the tear film?

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

Explanation

Topical drugs penetrate the cornea in a sequence through epithelium, stroma and endothelium to enter the anterior chamber. Corneal permeability is the principal route for anterior segment delivery.

Question 34Ocular Pharmacology and Therapeutics

A patient with asthma controlled on an inhaler requires a topical beta-blocker for glaucoma. Which choice is most appropriate?

  1. Betaxolol
  2. Brimonidine Correct answer.
  3. Timolol
  4. Levobunolol

Explanation

Nonselective beta-blockers can precipitate bronchospasm and should be avoided in reactive airway disease. Brimonidine, an alpha-2 agonist, avoids beta-mediated pulmonary effects and is a reasonable alternative.

Question 35Ocular Pharmacology and Therapeutics

Which topical mydriatic is a direct-acting parasympatholytic that also causes cycloplegia?

  1. Apraclonidine
  2. Phenylephrine
  3. Hydroxyamphetamine
  4. Tropicamide Correct answer.

Explanation

Tropicamide is an antimuscarinic agent that blocks the iris sphincter and ciliary muscle, producing both mydriasis and cycloplegia. Phenylephrine is a direct sympathomimetic that dilates without cycloplegia.

Question 36Ocular Pharmacology and Therapeutics

A 6-year-old needs cycloplegic refraction. Which agent provides the most reliable and prolonged cycloplegia?

  1. Phenylephrine 2.5%
  2. Tropicamide 1%
  3. Atropine 1% Correct answer.
  4. Cyclopentolate 1%

Explanation

Atropine produces the most profound and longest-lasting cycloplegia, which is valuable when full paralysis of accommodation is required, as in some paediatric refractions. Its slow recovery is the trade-off for this depth of effect.

Question 37Ocular Pharmacology and Therapeutics

Which mechanism best describes the anti-inflammatory action of topical corticosteroids in the eye?

  1. Stabilisation of mast cells
  2. Blockade of histamine H1 receptors
  3. Inhibition of cyclo-oxygenase by irreversible acetylation
  4. Inhibition of phospholipase A2 and reduced prostaglandin synthesis Correct answer.

Explanation

Corticosteroids act through glucocorticoid receptors to inhibit phospholipase A2 and downstream arachidonic acid pathways, reducing prostaglandin and leukotriene production. This broadly suppresses inflammatory mediator synthesis.

Question 38Ocular Pharmacology and Therapeutics

A patient on long-term topical dexamethasone develops raised intraocular pressure. Which mechanism explains this?

  1. Increased aqueous humour production by the ciliary body
  2. Pupillary block from posterior synechiae
  3. Reduced uveoscleral outflow from ciliary muscle relaxation
  4. Increased trabecular meshwork resistance to aqueous outflow Correct answer.

Explanation

Steroid-induced ocular hypertension results from increased resistance to aqueous outflow at the trabecular meshwork, often with extracellular matrix changes. Aqueous production is not the principal driver.

Question 39Ocular Pharmacology and Therapeutics

Which topical antibiotic is avoided in neonates and late pregnancy because it can cause kernicterus by displacing bilirubin from albumin?

  1. Ciprofloxacin
  2. Tobramycin
  3. Erythromycin
  4. Chloramphenicol Correct answer.

Explanation

Chloramphenicol displaces bilirubin from albumin binding sites, raising free bilirubin and the risk of kernicterus in neonates. This limits its use in that population.

Question 40Ocular Pharmacology and Therapeutics

Which agent is a topical mast cell stabiliser used prophylactically in allergic conjunctivitis?

  1. Oxymetazoline
  2. Sodium cromoglicate Correct answer.
  3. Ketorolac
  4. Prednisolone acetate

Explanation

Sodium cromoglicate stabilises mast cells and prevents degranulation, so it works best when used before allergen exposure. It has little immediate effect on an established reaction.

Question 41Ocular Pharmacology and Therapeutics

Which topical anaesthetic property makes proparacaine suitable for brief procedures such as tonometry?

  1. Irreversible nerve blockade
  2. Slow onset with prolonged anaesthesia
  3. Rapid onset with short duration of action Correct answer.
  4. Systemic absorption with minimal corneal effect

Explanation

Proparacaine produces rapid corneal anaesthesia that lasts only a short time, which suits quick office procedures. Its short duration avoids prolonged corneal insensitivity.

Question 42Ocular Pharmacology and Therapeutics

A patient instils two different topical drops at the same time. Which advice best improves ocular bioavailability of the second drop?

  1. Instil both drops simultaneously
  2. Wash the eye with saline between drops
  3. Wait about five minutes between the two drops Correct answer.
  4. Apply punctal occlusion before both drops

Explanation

Separating drops by about five minutes prevents the second drop from washing out the first and reduces additive systemic absorption. Simultaneous instillation shortens contact time and lowers efficacy.

Question 43Ocular Pharmacology and Therapeutics

Which property of the corneal epithelium most limits penetration of hydrophilic drugs?

  1. Its high water content
  2. Its lack of tight junctions
  3. Its continuous mitotic activity
  4. Its lipid-rich apical cell membranes Correct answer.

Explanation

The superficial epithelial cells have lipid-rich membranes, so hydrophilic drugs partition poorly and penetration is limited. This is why the epithelium favours passage of lipophilic compounds.

Question 44Ocular Pharmacology and Therapeutics

Which systemic side effect is characteristic of topical alpha-2 agonists such as brimonidine?

  1. Hyperkalaemia
  2. Bronchospasm
  3. Photosensitivity rash
  4. Dry mouth and fatigue Correct answer.

Explanation

Alpha-2 agonists can cause systemic effects including dry mouth, fatigue and drowsiness from central alpha-2 stimulation. These are common reasons for intolerance.

Question 45Ocular Pharmacology and Therapeutics

Which topical agent increases uveoscleral outflow by remodelling the extracellular matrix of the ciliary muscle?

  1. Timolol
  2. Dorzolamide
  3. Pilocarpine
  4. Latanoprost Correct answer.

Explanation

Prostaglandin analogues increase uveoscleral outflow, partly through matrix metalloproteinase-mediated remodelling of ciliary muscle extracellular matrix. This produces a sustained pressure reduction.

Question 46Ocular Pharmacology and Therapeutics

A patient develops conjunctival hyperaemia and iris darkening after months of a topical prostaglandin analogue. What is the best explanation?

  1. Acute allergic degranulation of mast cells
  2. Direct toxic damage to corneal endothelium
  3. Increased melanogenesis in iris melanocytes Correct answer.
  4. Beta-receptor mediated vasospasm

Explanation

Prostaglandin analogues increase melanin production in iris melanocytes, causing permanent iris darkening, and also cause conjunctival hyperaemia. These effects are class-typical and largely cosmetic.

Question 47Optics, Refraction and Clinical Optics

A thin convex lens has a focal length of 20 cm in air. What is its refractive power in dioptres?

  1. +0.05 D
  2. +20.00 D
  3. +2.00 D
  4. +5.00 D Correct answer.

Explanation

Refractive power in dioptres equals the reciprocal of the focal length in metres (P = 1/f). A focal length of 20 cm is 0.20 m, so P = 1/0.20 = +5.00 D. Convex lenses converge light and therefore carry a positive power.

Question 48Optics, Refraction and Clinical Optics

In a reduced eye model, the total power of the schematic eye is approximately:

  1. +20 D
  2. +43 D
  3. +80 D
  4. +60 D Correct answer.

Explanation

The reduced (schematic) eye has a total dioptric power of about +60 D, combining corneal power (~+43 D) and crystalline lens power (~+19 D). This total power brings parallel light to a focus on the retina.

Question 49Optics, Refraction and Clinical Optics

The far point of a myopic eye is 50 cm in front of the eye. Which spectacle correction at the spectacle plane (ignoring vertex distance) is required?

  1. +2.00 D
  2. -5.00 D
  3. -0.50 D
  4. -2.00 D Correct answer.

Explanation

A myope needs a diverging (minus) lens whose focal point coincides with the far point. The far point is 0.50 m away, so the required power is 1/0.50 = 2.00 D, and because it must diverge light it is -2.00 D.

Question 50Optics, Refraction and Clinical Optics

Snell's law governing refraction at an interface is correctly expressed as:

  1. n1 θ1 = n2 θ2
  2. n1 tan θ1 = n2 tan θ2
  3. n1 cos θ1 = n2 cos θ2
  4. n1 sin θ1 = n2 sin θ2 Correct answer.

Explanation

Snell's law states that the product of refractive index and the sine of the angle (measured to the normal) is constant across an interface. Hence n1 sin θ1 = n2 sin θ2 describes refraction correctly.