Human Eye — Set 3

Correct Answer: B. Providing nutrients and maintaining pressure

• **Cones** = cone-shaped photoreceptors concentrated in the fovea that are responsible for colour vision in bright light. • **Three types** (S, M, L cones) respond to short (~420 nm), medium (~530 nm), and long (~560 nm) wavelengths. • 💡 Wrong-option analysis: Rods: only one type — achromatic, for dim-light vision; Bipolar cells: relay neurons between photoreceptors and ganglion cells — not light sensitive; Ganglion cells: output neurons of the retina — carry signals, do not detect colour.

Correct Answer: C. Cylindrical lens

• **Vitamin A deficiency** = reduces rhodopsin synthesis in rods, impairing dim-light vision and causing night blindness (nyctalopia). • **Rhodopsin** requires 11-cis-retinal (derived from vitamin A/retinol) as its chromophore for light absorption. • 💡 Wrong-option analysis: Vitamin B12: essential for nerve function and red blood cells, not for rod pigment; Vitamin C: antioxidant, supports eye health generally but not rhodopsin; Vitamin D: regulates calcium/bone — not directly linked to night blindness.

Correct Answer: C. Cones

• **Glaucoma** = a group of eye diseases in which raised intraocular pressure (IOP > 21 mmHg) damages the optic nerve, leading to vision loss. • **Treatment** includes eye drops that reduce aqueous humour production or increase drainage; surgery in severe cases. • 💡 Wrong-option analysis: Cataract: clouding of the lens — no increase in IOP; Myopia: refractive error of the eyeball length; Astigmatism: corneal shape defect — no pressure involvement.

Correct Answer: C. 1/16 second

• **Cataract** = clouding or opacification of the crystalline lens due to protein denaturation, scattering incoming light and reducing visual clarity. • **Most common cause** is ageing; UV radiation and diabetes accelerate it; corrected by surgical lens replacement. • 💡 Wrong-option analysis: Glaucoma: optic nerve damage from high IOP — lens remains clear; Myopia: eyeball-length refractive error; Colour blindness: cone pigment deficiency — not a lens opacity.

Correct Answer: B. Conjunctiva

• **Yellow spot (macula lutea)** = a yellowish, oval region at the centre of the retina rich in cones, responsible for fine detail and colour vision. • **Diameter** ~5.5 mm; the fovea (highest acuity pit) lies at its centre. • 💡 Wrong-option analysis: Blind spot (optic disc): no photoreceptors — opposite of sharpest vision; Cornea: outer transparent dome — not a retinal region; Iris: the muscular diaphragm around the pupil.

Correct Answer: C. Sclera

• **Near point** = the closest point at which the eye can focus clearly; ~25 cm for a normal young adult eye. • **Power of accommodation** is greatest at the near point; the lens is at maximum curvature. • 💡 Wrong-option analysis: 50 cm: near point of a hypermetropic or older eye; Infinity: the far point (not near point) of a normal eye; 15 cm: closer than normal — possible only in young children with high accommodation.

Correct Answer: B. Optic nerve

• **Power of a lens** = P = 1/f (metres), measured in dioptres (D); a +2 D lens has a focal length of 0.5 m. • **Combination**: powers add algebraically — P_total = P1 + P2 — when lenses are in contact. • 💡 Wrong-option analysis: P = f: dimensionally wrong — power must be inverse of focal length; P = 1/f (cm): gives 100× too large a value; P = f² : has no physical basis in optics.

Correct Answer: A. Vitamin A

• **Far point** = the most distant point at which a relaxed eye can focus clearly; for a normal eye it is at infinity. • **In myopia**, the far point moves closer (e.g., 1 m), meaning the eye cannot focus on objects beyond that distance. • 💡 Wrong-option analysis: 25 cm: the near point (least distance of distinct vision), not the far point; 1 m: far point only of a myopic eye (not normal); Optic disc: a structural feature — not a focusing distance.

Correct Answer: B. 4 D

• **Colour blindness** = inability to distinguish certain colours, most commonly red-green, due to absent or defective cone pigments. • **Genetics**: X-linked recessive, so it affects ~8% of males and only ~0.5% of females. • 💡 Wrong-option analysis: Optic nerve defect: causes loss of all vision, not selective colour loss; Rod cell deficiency: leads to night blindness, not colour confusion; Corneal opacity: scatters all light — no colour selectivity.

Correct Answer: D. Pupil

• **Lens** = a transparent biconvex structure made of protein fibres (crystallins), enclosed in a capsule, that provides fine-tunable focusing power. • **Accommodation range** in a young eye: roughly 14–40 D total (cornea ≈ 43 D fixed + lens ~0–20 D variable). • 💡 Wrong-option analysis: Cornea: fixed-focus element providing ~43 D, no accommodation; Vitreous humour: gel filling the eye — no refractive change; Iris: controls light entry — not responsible for focusing.