Human Eye — Set 1
Physics · मानव नेत्र · Questions 1–10 of 60
Which part of the human eye gives it its visible color?
Correct Answer: B. Iris
• **Iris** = the coloured circular diaphragm that gives the eye its visible colour and controls pupil size. • **Pupil diameter** ranges from ~2 mm (bright light) to ~8 mm (dark) — iris muscles control this range. • 💡 Wrong-option analysis: Sclera: the white outer coat, provides structural support, not colour; Cornea: transparent front dome, no colour pigment; Retina: inner light-sensitive layer at the back, not visible externally.
The image formed on the retina of a normal human eye is generally?
Correct Answer: B. Real, inverted, diminished
• **Real, inverted, diminished** = the eye lens (a convex lens) refracts light to form a real image on the retina. • **Distance from lens to retina** is ~2.5 cm — far shorter than object distance, so the image is diminished. • 💡 Wrong-option analysis: Virtual erect magnified: only a virtual object or diverging lens gives this; Virtual inverted diminished: concave mirrors can give this, not a simple convex lens in normal use; Real erect magnified: not possible for a real image through a simple convex lens when object is beyond F.
Which photoreceptor cells are mainly responsible for vision in dim light?
Correct Answer: B. Rods
• **Rods** = rod-shaped photoreceptors that contain rhodopsin and are highly sensitive to low-light intensities. • **~120 million rods** exist in the human retina, concentrated in the peripheral region outside the fovea. • 💡 Wrong-option analysis: Ganglion cells: relay nerve signals to the brain, not photoreceptors; Cones: need bright light for colour vision, insensitive in dim conditions; Ciliary muscles: adjust lens curvature for focus, not light detection.
A concave lens is commonly used to correct which defect of vision?
Correct Answer: A. Myopia
• **Myopia (short-sightedness)** = parallel rays from distant objects converge in front of the retina due to an elongated eyeball or too-powerful lens. • **Concave lens** with negative focal length diverges the rays so they now converge on the retina. • 💡 Wrong-option analysis: Hypermetropia: near-point too far away, corrected by convex lens; Presbyopia: age-related loss of accommodation, managed with bifocals; Cataract: clouding of the lens, treated surgically — not corrected by a spectacle lens.
In the human eye, most of the refraction of light occurs at the?
Correct Answer: B. Cornea
• **Convex lens** = converges rays to bring near objects, whose diverging rays would otherwise focus behind the retina, onto the retina. • **Power** of corrective lens is positive (e.g., +2 D for moderate hypermetropia). • 💡 Wrong-option analysis: Concave lens: diverges light, moves focus further back — worsens hypermetropia; Bifocal lens: used for presbyopia (both myopia and hypermetropia simultaneously); Cylindrical lens: corrects astigmatism, not the spherical error of hypermetropia.
Aqueous humor is primarily found in the space between the?
Correct Answer: C. Cornea and lens
• **Ciliary muscles** = ring of smooth muscle behind the iris that alters the curvature of the elastic lens for near or distant focus. • **Accommodation range**: lens becomes nearly spherical (f ≈ 7 cm) for near vision and flat (f ≈ 17 cm) for far vision. • 💡 Wrong-option analysis: Iris: controls pupil diameter, not lens shape; Retina: the photosensitive screen — does not move or bend; Cornea: fixed-curvature transparent dome — provides ~70% of total eye refraction but does not change shape for focus.
In bright light, the pupil becomes smaller mainly due to the action of the?
Correct Answer: C. Circular muscles of the iris
• **Blind spot** = the optic disc where the optic nerve exits the eye — it has no photoreceptors, so no image is formed there. • **Location**: ~15° to the nasal side of the fovea; objects falling on it are completely invisible. • 💡 Wrong-option analysis: Fovea: highest cone density, sharpest colour vision — the opposite of a blind area; Iris: controls light entry, not image formation; Cornea: the outer transparent layer, not a part of the retina.
The least distance of distinct vision for a normal adult human eye is about?
Correct Answer: A. 25 cm
• **Rhodopsin** = a light-sensitive pigment (opsin + retinal) in rod cells that decomposes on absorbing light, triggering a nerve impulse. • **Regeneration** requires vitamin A; deficiency leads to night blindness (nyctalopia). • 💡 Wrong-option analysis: Melanin: provides eye colour in the iris/choroid, unrelated to night vision; Iodopsin: the cone pigment for colour vision in bright light, not dim-light vision; Haemoglobin: oxygen-carrying blood protein — completely unrelated to photoreception.
The blind spot in the human eye occurs because the retina at that point has?
Correct Answer: C. No photoreceptors
• **Presbyopia** = age-related hardening of the lens that reduces its ability to change shape, so both near and far focus are affected. • **Onset** typically after age 40; power of near-reading correction is usually +1 to +3 D. • 💡 Wrong-option analysis: Myopia: excess converging power of the eye, occurs at any age; Hypermetropia: structural — eyeball too short; Astigmatism: unequal curvature of cornea or lens, not age-specific.
The part of the retina responsible for the sharpest vision is the?
Correct Answer: A. Fovea (yellow spot)
• **Fovea centralis** = a small depression in the retina packed almost entirely with cones, giving the sharpest, most colourful central vision. • **Visual acuity** is highest here; cones are densest (~150,000 per mm²) and each connects to a single nerve fibre. • 💡 Wrong-option analysis: Blind spot: optic disc with no receptors, no vision; Cornea: the outer transparent dome, not inside the retina; Pupil: aperture controlling light entry, no photoreceptors.