Human Eye — Set 6

Correct Answer: D. Sclera

• **Refraction at the cornea** = incoming light bends at the air–cornea interface because the refractive index changes from 1.0 (air) to ~1.376 (cornea). • **Cornea accounts for ~43 D** of the eye's total ~60 D refracting power. • 💡 Wrong-option analysis: Reflection: light bouncing back — not how images form inside the eye; Diffraction: bending around obstacles — negligible at corneal scale; Absorption: some UV is absorbed by cornea, but this does not form an image.

Correct Answer: C. Mostly cones and almost no rods

• **Rhodopsin bleaching** = on absorbing a photon, 11-cis-retinal isomerises to all-trans-retinal, changing the shape of opsin and triggering a nerve signal. • **Recovery (dark adaptation)** takes ~20–30 minutes as rhodopsin is resynthesised — hence poor vision when moving from bright to dark. • 💡 Wrong-option analysis: Melanin release: triggered by UV on skin — not the retinal light-response mechanism; Iodopsin activation: happens in cones for colour vision, not rods; Vitamin C oxidation: antioxidant reaction — not the photoreceptor signal cascade.

Correct Answer: A. Mydriasis

• **Dioptre** = the SI unit of lens power; 1 D = power of a lens with focal length of 1 metre. • **Formula**: P (D) = 1 / f (m); a 50 cm focal length lens has P = +2 D; a concave lens of –3 D has f = –0.33 m. • 💡 Wrong-option analysis: Metre: unit of focal length — not of power; Candela: unit of luminous intensity — unrelated to lens power; Lux: unit of illuminance — not optical power.

Correct Answer: D. Radial muscles

• **Photopic vision** = bright-light colour vision mediated by cone cells; scotopic vision = dim-light achromatic vision by rod cells. • **Threshold**: cones require illuminance > ~0.01 cd/m²; rods saturate above this level and cone vision takes over. • 💡 Wrong-option analysis: Both rods and cones simultaneously: this describes mesopic vision (twilight) — not purely photopic; Only rods: this describes scotopic, not photopic; Ganglion cells: relay neurons — not the mediators of bright-light vision.

Correct Answer: D. Thin membrane lining eyelids and covering sclera

• **Aqueous humour** = a watery fluid in the anterior segment (between cornea and lens) that nourishes the avascular cornea and lens. • **Composition**: mostly water, glucose, amino acids, and ascorbic acid; normal IOP maintained at 10–21 mmHg. • 💡 Wrong-option analysis: Vitreous humour: posterior gel-filled chamber (behind lens) — not anterior; Lacrimal fluid: external tear film on the cornea surface, not inside the eye; Blood: circulates in the choroid — not inside the anterior chamber in a healthy eye.

Correct Answer: B. Help maintain the spherical shape of the eyeball

• **Accommodation reflex** = a triad: convergence of eyes (medial recti contract), pupil constriction, and lens thickening — all triggered by looking at a near object. • **Pupil constriction** during near focus reduces aberrations and increases depth of field. • 💡 Wrong-option analysis: Pupil dilation only: happens in dim light — opposite of the near reflex; Eye divergence: occurs when looking far away — opposite of near convergence; Lens flattening: happens when focusing far, not near.

Correct Answer: B. Red–green discrimination

• **Binocular vision** = simultaneous use of both eyes to form a single, fused visual image with depth perception (stereopsis). • **Binocular visual field overlap** is ~120° in humans — this overlap zone enables stereoscopic depth computation. • 💡 Wrong-option analysis: Monocular vision: one eye only — no stereopsis; Peripheral vision: the outer parts of the visual field, not the overlapping central zone; Colour vision: dependent on cones — independent of whether one or two eyes are used.

Correct Answer: D. 1 minute of arc

• **Optic disc** = the point where the optic nerve exits the retina; has no photoreceptors, creating the physiological blind spot. • **Location**: ~15° nasal to the fovea; diameter ~1.5 mm; normally has a pale central cup. • 💡 Wrong-option analysis: Fovea: highest visual acuity — the opposite of a blind spot; Macula: the yellow-pigmented central retinal area with densest cones; Cornea: the transparent front dome — not inside the retina.

Correct Answer: C. An aperture in the iris

• **Contact lenses** = thin corrective lenses worn directly on the corneal surface; they correct refraction by altering the effective radius of curvature of the cornea. • **Types**: soft (hydrogel/silicone hydrogel) for comfort; rigid gas-permeable for better optics and astigmatism correction. • 💡 Wrong-option analysis: Spectacles: worn away from the eye surface — separate optical system; Intraocular lens (IOL): surgically implanted inside the eye after cataract removal — not worn externally; Corneal transplant: a surgical graft — not a wearable corrective device.

Correct Answer: D. Many nerve endings

• **Pupillary light reflex** = the automatic constriction of the pupil in response to bright light, mediated by the optic nerve (CN II) and oculomotor nerve (CN III). • **Consensual reflex**: light in one eye causes constriction in both eyes — used clinically to test brainstem integrity. • 💡 Wrong-option analysis: Accommodation reflex: pupil constriction occurs during this too, but it is triggered by near objects, not light; Corneal reflex: a blink response to corneal touch — not a pupil response; Vestibulo-ocular reflex: stabilises gaze during head movement — not light-driven.