Human Eye — Set 4

Correct Answer: C. Preventing internal reflection by absorbing stray light

• **Snell's law** at the eye: n₁ sin θ₁ = n₂ sin θ₂ — refraction at the cornea and lens surfaces bends light to focus on the retina. • **Refractive indices**: air = 1.0, cornea ≈ 1.376, aqueous/vitreous ≈ 1.336, lens ≈ 1.40. • 💡 Wrong-option analysis: Reflection is how light bounces off a surface — it does not create images inside the eye; Diffraction: bending around edges, important only in microscopy; Dispersion: splitting into colours — corrected for in the eye but not the primary focusing mechanism.

Correct Answer: A. Biconvex and transparent

• **Hypermetropia (far-sightedness)** = near objects cannot be focused on the retina because either the eyeball is too short or the lens is too weak. • **Correction**: convex (converging) lens with positive power brings near-object rays to converge on the retina. • 💡 Wrong-option analysis: Concave lens: diverges rays, worsening the underpowered eye; Cylindrical lens: for astigmatism; Bifocal: for presbyopia needing both near and far correction simultaneously.

Correct Answer: C. There is a large refractive index change at the air–cornea boundary

• **Tears (lacrimal fluid)** = secreted by lacrimal glands, tears lubricate and protect the cornea, wash away debris, and contain lysozyme (antibacterial enzyme). • **pH** ~7.4; protein concentration ~0.7% with immunoglobulins providing immune defence. • 💡 Wrong-option analysis: Aqueous humour: fills the anterior chamber of the eye, not the surface; Sebaceous secretion: oils from skin glands — not from lacrimal glands; Mucus: secreted by goblet cells in the conjunctiva, not from the lacrimal gland.

Correct Answer: A. Convex lens

• **Retina** = the innermost, photosensitive neural layer lining the back of the eyeball, containing rods, cones, bipolar cells, and ganglion cells. • **Thickness** ~0.1–0.5 mm; ~130 million photoreceptors (120M rods + 6–7M cones) convert light to nerve impulses. • 💡 Wrong-option analysis: Cornea: the outermost transparent dome — no photoreceptors; Lens: focuses light but detects nothing; Choroid: vascular supply layer behind the retina — not itself photosensitive.

Correct Answer: B. Reduced accommodation due to aging

• **Accommodation** = the automatic adjustment of the eye's focal length by changing lens curvature via ciliary muscle contraction or relaxation. • **For near objects**: ciliary muscles contract → zonular fibres relax → lens becomes more convex → shorter focal length. • 💡 Wrong-option analysis: Changing pupil size: adjusts brightness, not focal length; Moving the retina: the retina is fixed — it does not move; Changing corneal curvature: the cornea is rigid — it does not flex during accommodation.

Correct Answer: A. Rhodopsin

• **Iris** = a thin circular muscular diaphragm with two sets of muscles (sphincter and dilator pupillae) that control the size of the pupil. • **Sphincter pupillae** contracts the pupil in bright light; **dilator pupillae** widens it in dim light — both are involuntary. • 💡 Wrong-option analysis: Sclera: rigid white outer coat — no muscular control; Choroid: vascular layer — carries blood, not a muscular diaphragm; Ciliary body: controls lens shape for accommodation, not pupil aperture.

Correct Answer: B. +3 D

• **Night blindness (nyctalopia)** = inability to see well in dim light due to rhodopsin deficiency in rod cells, commonly from vitamin A deficiency. • **Vitamin A** is required to synthesise 11-cis-retinal, the chromophore of rhodopsin; supplementation reverses the condition. • 💡 Wrong-option analysis: Vitamin C deficiency: causes scurvy — not night blindness; Vitamin D deficiency: causes rickets — no link to night vision; Vitamin B1 deficiency: causes beriberi — not specifically a vision problem.

Correct Answer: D. Canal of Schlemm

• **Conjunctiva** = a thin transparent mucous membrane that lines the inner surface of the eyelids and covers the white of the eye (sclera). • **Goblet cells** in the conjunctiva secrete mucus that forms the inner layer of the tear film. • 💡 Wrong-option analysis: Sclera: the structural white coat beneath the conjunctiva; Cornea: the front transparent dome — not a mucous membrane; Choroid: vascular middle layer inside the eyeball.

Correct Answer: C. In front of the retina

• **Intraocular pressure (IOP)** = pressure exerted by aqueous humour inside the eye; normal range ~10–21 mmHg. • **Elevated IOP** (> 21 mmHg) compresses the optic nerve fibres at the optic disc, causing glaucoma-related vision loss. • 💡 Wrong-option analysis: Vitreous pressure: the gel pressure is passive — it does not flow and cause clinical pressure elevation; Arterial blood pressure: systemic, measured differently — not what IOP means; Lens tension: lens is elastic, not a fluid-pressure source.

Correct Answer: A. Behind the retina

• **Ciliary body** = a ring of tissue behind the iris that contains ciliary muscles (for accommodation) and ciliary processes (that secrete aqueous humour). • **Aqueous humour** production rate ~2 µL/min; it flows from the posterior chamber through the pupil to the anterior chamber. • 💡 Wrong-option analysis: Lens: the focusing element — does not produce fluids; Sclera: outer coat — not a secretory tissue; Choroid: supplies blood but does not secrete aqueous humour.