NDA (II) 2017 General Ability Test · Question 129 of 150

Correct — A, the claim that the eye contains a diverging lens. The first half of that sentence is right and the second half is wrong. The eye is indeed a refracting system, but every refracting element in it converges light: the cornea does most of the bending, and the crystalline lens behind the pupil is a convex, converging lens whose curvature the ciliary muscles adjust to focus near and far objects. A diverging lens spreads rays apart and cannot form a real image on a screen, so an eye built around one could never throw a focused picture onto the retina. Diverging lenses do appear in eyewear — spectacles for short sight use them — but that is a correction placed in front of the eye, not a part of it.

• (b)The retina of the human eye contains millions of light sensitive cells, called rods and cones, which convert the light into electrical messages. — This is correct. Rods handle dim light and give no colour information; cones work in bright light and give colour vision. Both are photoreceptors that turn the incoming light into nerve impulses carried to the brain by the optic nerve.
• (c)Every image that is focused on the retina is upside down. — Also correct. A converging lens forms a real, inverted image, so the picture on the retina is genuinely upside down and laterally reversed. The brain does the reinterpreting, which is why we do not notice.
• (d)We need both eyes to judge the relative positions of objects accurately. — Correct as well. The two eyes view a scene from slightly different positions, and the brain uses that disparity to build a sense of depth. Close one eye and threading a needle becomes noticeably harder.

The eye works like a camera with an adjustable lens. Light refracts first at the cornea, passes through the pupil whose size the iris controls, and is brought to a focus by the crystalline lens onto the retina. The lens is convex and its focal length is changed by the ciliary muscles, an ability called accommodation. The retina carries rods for low light and cones for colour, and the optic nerve carries the signal onward. The image formed is real, inverted and smaller than the object.

Not-correct items on the eye almost always attack one of three points: the type of lens, the nature of the retinal image, or the role of the two eyes. Fix the first and the rest follow. Any optical system that forms a real image on a screen must converge light, so the eye's lens has to be convex. It helps to keep the defects straight too: myopia is corrected with a concave (diverging) lens, hypermetropia with a convex one, and presbyopia is the age-related stiffening that costs the lens its power to accommodate.

• The crystalline lens of the eye is convex and converging; most of the refraction actually happens at the cornea.
• Accommodation is the change of the lens's focal length by the ciliary muscles.
• The image on the retina is real, inverted and smaller than the object.
• Rods give vision in dim light; cones give colour vision in bright light.
• Concave lenses correct short sight and convex lenses correct long sight — they are worn in front of the eye, not built into it.

• Confusing the diverging lens in spectacles for short sight with the converging lens inside the eye.
• Assuming the lens does most of the focusing; the cornea contributes the larger share of the refraction.
• Thinking the brain receives an upright image — the inversion happens on the retina and is undone in the brain.

The eye is asked as a not-correct statement item, as a defects-and-correcting-lens pairing, or as a question on accommodation and the near point.

• practice — not a real PYQ

  The defect of vision in which a person cannot see distant objects clearly is corrected using

  • (a)a convex lens
  • (b)a concave lens
  • (c)a cylindrical lens
  • (d)a plane mirror

  Answer(b) a concave lens — short sight forms the image in front of the retina, and a diverging lens pushes it back.

• practice — not a real PYQ

  The ability of the eye to focus on objects at different distances by changing the focal length of its lens is called

  • (a)persistence of vision
  • (b)accommodation
  • (c)resolution
  • (d)adaptation

  Answer(b) accommodation — the ciliary muscles alter the curvature of the crystalline lens.