Myopia and Hypermetropia can be respectively corrected by
- (1)Plano-concave and Cylindrical lens
- (2)Convex and Concave lens
- (3)Concave and Convex lens
- (4)Concave and Cylindrical lens
Answer
Why
Correct — option (3), Concave and Convex lens.
In myopia (near-sightedness) a person sees nearby objects clearly but not distant ones, because the image of a distant object forms in front of the retina. NCERT's Class 10 Science says: "This defect can be corrected by using a concave lens of suitable power."
In hypermetropia (far-sightedness) distant objects are clear but nearby ones are not, because light from a close object is focused at a point behind the retina. For this NCERT says: "This defect can be corrected by using a convex lens of appropriate power."
The stem names myopia first and hypermetropia second, and asks for the lenses "respectively", so the pair is Concave and Convex lens.
The idea to remember: myopia needs a concave (diverging) lens; hypermetropia needs a convex (converging) lens.
Why the others are wrong
- (1)Plano-concave and Cylindrical lens — The first half could serve for myopia: a plano-concave lens has one flat and one concave face and, in air, diverges light like any concave lens.
The second half fails. A cylindrical lens focuses light into a line rather than a point, and it is prescribed to correct astigmatism, not hypermetropia.
- (2)Convex and Concave lens — These are the right two lenses in the wrong order. A convex lens corrects hypermetropia and a concave lens corrects myopia, so this pair would answer the stem only if it listed hypermetropia first and myopia second.
The stem asks for myopia first, then hypermetropia, respectively.
- (4)Concave and Cylindrical lens — Concave is right for myopia, but a cylindrical lens corrects astigmatism, in which the cornea or eye lens is curved more in one direction than the other, blurring vision at every distance.
Hypermetropia needs a convex, converging lens that adds focusing power so the image forms on the retina.
Concept
The eye lens focuses light on the retina. Its ability to change focal length for near and distant objects is accommodation; for a young adult with normal vision, the near point is about 25 cm.
In myopia the image of a distant object forms in front of the retina, because of excessive curvature of the eye lens or an elongated eyeball. A diverging (concave) lens brings the image back onto the retina.
In hypermetropia light from a nearby object is focused behind the retina, because the eye lens's focal length is too long or the eyeball too small. A converging (convex) lens adds the focusing power needed.
In air, concave lenses diverge light and convex lenses converge it.
RPSC's 2023 syllabus lists "Basics of Everyday Science." under Science & Technology, and spectacle lenses apply the ray optics of lenses to the human eye.
NCERT names three common refractive defects of vision: myopia, hypermetropia and presbyopia. It says they can be corrected with suitable spherical lenses, and also with contact lenses or surgery.
A person with both myopia and hypermetropia may need bifocal lenses, which NCERT describes as concave in the upper part for distant vision and convex in the lower part for near vision.
Key facts
- Myopia: the image of a distant object forms in front of the retina; a concave lens corrects it (NCERT Class 10 Science).
- Myopia can arise from excessive curvature of the eye lens or elongation of the eyeball.
- Hypermetropia: light from a nearby object focuses behind the retina, from a too-long focal length of the eye lens or a too-small eyeball; a convex lens corrects it.
- Presbyopia arises from weakening ciliary muscles and a less flexible eye lens with age.
- Cylindrical lenses are prescribed to correct astigmatism, where the cornea or lens curves more in one direction than another.
Sources: NCERT Class 10 Science, "The Human Eye and the Colourful World"; astigmatism and cylindrical lens from their Wikipedia articles.
Study next
Common traps
- Reading the names backwards: a near-sighted (myopic) person sees near objects clearly; the trouble is with distant objects.
- Ignoring "respectively": convex-then-concave has the right lenses in the wrong order for myopia-then-hypermetropia.
- Shape names hide lens type: a plano-concave lens is still a diverging lens, while a cylindrical lens is the one for astigmatism.
A question can name a defect and ask for the corrective lens, or describe where the image forms and ask for the defect.
A question can also ask the causes of a defect, or which lens forms the upper and lower parts of a bifocal.
Related PYQs
In cataract the natural lens becomes cloudy because of change in which of the following ?
- (1) Protein
- (2) Carbohydrate
- (3) Fat
- (4) Tear glands
Answer(1)
Same organ, a different defect. That question asks what change makes the eye's natural lens cloudy in cataract (RPSC's key: protein); this one asks which spectacle lenses correct myopia and hypermetropia.
Practice
- practice — not a real PYQ
In a common type of bifocal lens described in NCERT Class 10 Science, the upper portion and the lower portion are respectively
- (a)convex and concave
- (b)concave and convex
- (c)cylindrical and convex
- (d)concave and cylindrical
Answer(2) — The upper concave part is for distant vision and the lower convex part for near vision. Option (1) reverses them, and options (3) and (4) bring in a cylindrical lens, which corrects astigmatism. - practice — not a real PYQ
According to NCERT Class 10 Science, hypermetropia can arise because
- (a)the eyeball has become elongated
- (b)the eye lens has excessive curvature
- (c)the eyeball has become too small
- (d)the cornea curves more in one direction than another
Answer(3) — NCERT gives a too-long focal length of the eye lens or a too-small eyeball as the causes of hypermetropia. Options (1) and (2) are causes of myopia, and option (4) describes astigmatism.