Human Eye — Set 2

Correct Answer: A. A jelly-like substance behind the lens

• **Astigmatism** = unequal curvature of the cornea (or lens) in different meridians causes rays in different planes to focus at different points. • **Correction**: a cylindrical (toric) lens whose curvature compensates the irregular corneal surface. • 💡 Wrong-option analysis: Concave lens: corrects myopia, not the meridional aberration of astigmatism; Convex lens: corrects hypermetropia; Bifocal lens: addresses simultaneous near/far defects of presbyopia.

Correct Answer: B. -0.5 D

• **Aqueous humour** = a clear, watery fluid secreted by the ciliary body that fills the anterior chamber between the cornea and the lens. • **Intraocular pressure** is maintained by its production and drainage; blockage causes glaucoma (IOP > 21 mmHg). • 💡 Wrong-option analysis: Vitreous humour: the thick gel behind the lens filling the main eyeball cavity; Choroid: the vascular pigmented layer — not a fluid; Lens: the transparent biconvex solid structure — not a fluid.

Correct Answer: A. An eyeball that is too short

• **Choroid** = a thin, heavily pigmented, highly vascular layer between the sclera and retina that nourishes the retina and absorbs stray light. • **Thickness** ~0.1–0.2 mm; its dark pigment prevents internal reflections that would blur vision. • 💡 Wrong-option analysis: Sclera: the tough white fibrous outer coat — provides structural rigidity, not blood supply; Retina: the innermost light-sensitive neural layer — not the vascular supply layer; Cornea: the clear front surface — has no blood vessels at all.

Correct Answer: A. +2 D

• **25 cm** = the least distance of distinct vision for a normal human eye — closest point at which the eye can focus without strain. • **Power of accommodation** at this point requires the lens to bulge to its maximum, about +4 D extra. • 💡 Wrong-option analysis: 10 cm: too close — lens cannot provide enough extra power in a normal eye; 50 cm: the near-point of a presbyopic or hypermetropic eye, not normal; 100 cm: the near-point of a severely hypermetropic eye.

Correct Answer: C. Eye lens becomes cloudy

• **Cornea** = the transparent, avascular, curved front surface of the eye that provides ~70% (~43 D) of the eye's total refractive power. • **Refractive index** of corneal tissue ≈ 1.376; its curvature (radius ~5.6 mm) determines most of the eye's focusing. • 💡 Wrong-option analysis: Iris: controls pupil size — no optical refraction; Lens: provides remaining ~30% of refraction and handles accommodation; Retina: the image screen — no refractive function.

Correct Answer: B. Increased intraocular pressure that can damage the optic nerve

• **Persistence of vision** = the retina retains an image for ~1/16 second after the stimulus is removed, creating an illusion of continuity. • **Film projection** at 24 frames/second exploits this — each frame persists long enough to merge with the next. • 💡 Wrong-option analysis: 1/8 second: too long — at this persistence, motion would appear blurred/doubled; 1/32 second: too short — flicker would be noticeable; 1/64 second: far too short, well below the actual measured threshold.

Correct Answer: A. Change the focal length of the eye lens

• **Vitreous humour** = a transparent, gel-like substance (mostly water + collagen + hyaluronic acid) that fills the large posterior chamber of the eyeball. • **Volume** ~4 mL; it maintains the spherical shape of the eyeball and transmits light without scattering. • 💡 Wrong-option analysis: Aqueous humour: the thin watery fluid in the anterior chamber (front of lens), not the main posterior cavity; Lens: the solid biconvex transparent structure; Cornea: the outermost transparent dome — not a gel filling.

Correct Answer: D. Reciprocal of focal length in meters

• **Sclera** = the tough, opaque, white fibrous outer coat of the eyeball that gives it structural rigidity and protects the inner tissues. • **Thickness** varies from ~1 mm at the back to ~0.5 mm at the equator; it constitutes ~80% of the eye's outer surface. • 💡 Wrong-option analysis: Cornea: transparent front dome (part of outer coat but not white/opaque); Choroid: middle vascular layer, dark-pigmented; Retina: innermost neural layer — not a structural outer coat.

Correct Answer: D. Near point and far point

• **Optic nerve** = a bundle of ~1 million nerve fibres that transmits electrical signals from retinal ganglion cells to the brain's visual cortex. • **Exit point** (optic disc) is located ~3 mm to the nasal side of the fovea and forms the blind spot. • 💡 Wrong-option analysis: Ciliary nerve: controls ciliary muscles and iris — does not carry visual information; Trigeminal nerve: sensation of face/eye surface — not vision; Oculomotor nerve: controls eye movement muscles — not image signal transmission.

Correct Answer: B. Light entering it is mostly absorbed inside the eye

• **Pupil** = the aperture (opening) in the centre of the iris through which light enters the eye. • **Diameter range**: ~2 mm (bright light) to ~8 mm (dark) — regulates light intensity on the retina by a factor of ~16. • 💡 Wrong-option analysis: Cornea: the transparent outer dome — not an aperture; Lens: the internal focusing element behind the iris; Iris: the muscular ring surrounding the pupil — it controls the pupil but is not itself the hole.