Ramesh cannot see distinctly objects kept beyond 2 m. This defect can be corrected by using a lens of power
- A.+0.5 D
- B.−0.5 D
- C.+0.2 D
- D.−0.2 D
Correct answer
Explanation
The correct answer is B, −0.5 D. Ramesh is short-sighted with a far point of 2 m, so he needs a concave lens of focal length −2 m, whose power is 1/(−2) = −0.5 dioptre.
In myopia, or short-sightedness, a person sees near objects clearly but distant ones look blurred, because the image of a far object forms in front of the retina. The eyeball may be too long or the eye lens too curved. A concave (diverging) lens corrects it by making rays from a distant object appear to come from the far point. For that, the focal length of the lens equals the far point distance with a minus sign. Power in dioptres is 1 divided by the focal length in metres.
A is wrong because a positive power means a convex lens, which corrects long-sightedness, not this defect.
C is wrong because it is a convex lens and its value comes from wrongly using 1/5 instead of 1/2.
D is wrong because −0.2 D would suit a far point of 5 m, not 2 m.
Exam tip: myopia needs a concave lens with f = −(far point); power P = 1/f in metres.
Practice Questions
View allIf one set of chromosomes for a given plant is represented as N; in case of double fertilization, the zygote and the endosperm nucleus of a diploid plant would have how many sets of chromosomes respectively?
- A.N and 2N
- B.2N and 2N
- C.N and 3N
- D.2N and 3N
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The zygote is diploid (2N) and the primary endosperm nucleus is triploid (3N). Double fertilisation is a feature of flowering plants.
Read the full explanation →Which one of the following statements is NOT correct for the given reaction? Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
- A.Iron is the reducing agent
- B.The solution turns green in colour after the reaction
- C.Copper is a more reactive metal than iron
- D.The reaction is an example of a redox reaction
Show answer
The reaction happens because iron is more reactive than copper and pushes it out of its salt, so statement C is false.
Read the full explanation →Which one of the following statements is correct ?
- A.Xylem consists of tracheids, vessels, xylem parenchyma and xylem fibres.
- B.Flexibility in plants is due sclerenchyma.
- C.Parenchyma have no intercellular spaces.
- D.Xylem consists of sieve plate, sieve tube and companion cells.
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These four elements make up the complex tissue that carries water and minerals upward from the roots. Tracheids and vessels are long, dead, thick-walled tubes through which water moves; xylem parenchyma is the only living part and stores food; xylem fibres give support.
Read the full explanation →When light is scattered by a molecule and the frequency of the scattered light is changed, this phenomenon is called
- A.Rayleigh scattering.
- B.Raman effect.
- C.Photoelectric effect.
- D.Rutherford scattering.
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In the Raman effect, a small part of the light scattered by molecules comes out with a changed frequency. C.V.
Read the full explanation →Which one of the following is not an example of a redox reaction?
- A.AlCl3 + 3H2O → Al(OH)3 + 3HCl
- B.2NaH → 2Na + H2
- C.4Fe + 3O2 → 2Fe2O3
- D.CuSO4 + Zn → Cu + ZnSO4
Show answer
This is a hydrolysis reaction in which no element changes its oxidation number, so nothing is oxidised or reduced. A redox reaction is one in which one substance loses electrons (its oxidation number rises) and another gains them (its oxidation number falls).
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