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General ScienceMediumAsked in: NDA · NDA-I 2017, GAT

If the absolute refractive indices of glass and water are 3/2 and 4/3 respectively, what will be the ratio of velocity of light in glass and water ?

  1. A.3 : 4
  2. B.4 : 3
  3. C.8 : 7
  4. D.8 : 9

Correct answer

D. 8 : 9

Explanation

The correct answer is D, 8 : 9. The speed of light in a medium is inversely proportional to its refractive index, so the ratio of speeds is the inverse ratio of the refractive indices.

The absolute refractive index of a medium is n = c/v, where c is the speed of light in vacuum and v its speed in the medium. So v = c/n. For glass, v(glass) = c ÷ 3/2 = 2c/3, and for water, v(water) = c ÷ 4/3 = 3c/4. The ratio v(glass) : v(water) = (2/3) : (3/4) = 8 : 9. Light therefore travels slower in glass, which is optically denser than water.

A is wrong: 3 : 4 is the inverse ratio of water's index alone and does not compare the two media. B is wrong: 4 : 3 is just the refractive index of water written as a ratio. C is wrong: 8 : 7 does not follow from the formula at all.

Exam tip: v₁ : v₂ = n₂ : n₁; a higher refractive index means a slower speed of light.

Practice Questions

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Q1.General ScienceAsked in: CDS · CDS-II 2020, GKEasy

The valency of phosphorus is

  1. A.2, 3
  2. B.3, 4
  3. C.4, 5
  4. D.3, 5
Show answer
Correct answer: D. 3, 5

Phosphorus has five electrons in its outer shell and shows two common valencies, 3 and 5. Phosphorus, atomic number 15, has the electronic configuration 2, 8, 5.

Q2.General ScienceAsked in: CDS · CDS-II 2026, GKMedium

Which one of the following statements is NOT correct ?

  1. A.Single celled eukaryotes belong to Kingdom Protista
  2. B.Yeast, a fungus is multicellular
  3. C.The fungi constitute a unique kingdom of heterotrophs
  4. D.Ascomycetes, a saprophytic fungi is multicellular or rarely unicellular
Show answer
Correct answer: B. Yeast, a fungus is multicellular

This statement is false, because yeast is one of the few fungi that are unicellular. In Whittaker's five-kingdom system, fungi form a separate kingdom.

Q3.General ScienceAsked in: CDS · CDS-II 2017, GKMedium

Desalination of seawater is done by using reverse osmosis. The pressure applied to the solution is

  1. A.larger than osmotic pressure
  2. B.smaller than osmotic pressure
  3. C.equal to osmotic pressure
  4. D.equal to atmospheric pressure
Show answer
Correct answer: A. larger than osmotic pressure

Only a pressure greater than the osmotic pressure of seawater can push pure water backwards through the membrane. In ordinary osmosis, water moves through a semipermeable membrane from the dilute side to the concentrated side.

Q5.General ScienceAsked in: CDS · CDS-II 2017, GKEasy

The foul smell of urine of a healthy man having healthy food, when spilled on floor, is mainly due to the bacterial decomposition of

  1. A.urea into sulphur dioxide
  2. B.sugar into carbon dioxide
  3. C.lipids into methane
  4. D.urea into ammonia
Show answer
Correct answer: D. urea into ammonia

Bacteria on the floor break down the urea in urine and release ammonia, which has a sharp, pungent smell. Urea, CO(NH2)2, is the main nitrogenous waste of humans.