UPSC Prelims 1995 · General Studies

Question 109: Science and Technology

Assertion (A): A diamond sparkles more than a glass imitation cut to the same shape. Reason (R): The refractive index of diamond is less than that of glass. In the context of the above two statements, which one of the following is correct?

aBoth A and R are true, and R is the correct explanation of A

bBoth A and R are true, but R is not a correct explanation of A

cA is true, but R is false

dA is false, but R is true

AnswerThe answer is (c).

A diamond has a very high refractive index, about 2.4, against about 1.5 for ordinary glass. A higher refractive index gives a smaller critical angle, so light entering a well-cut diamond is totally internally reflected many times before it emerges, making it sparkle. The Assertion is therefore true, but the Reason, which says diamond's index is lower, is false. Remember: a diamond sparkles because of its high refractive index.

A. Diamond sparkles more than a glass imitation of the same cut.
R. Diamond's refractive index is higher than glass, not lower.

How you could have got this

If you were sure statement 1 is correct, (d) goes. Then judge the second statement; if it is correct too, decide whether it explains the first.

This follows an unofficial answer: UPSC has not published its key for this year, and if that answer is wrong, so is this path.

Which options go is worked out from UPSC’s key and the options, not by a model.

Why each option is right or wrong

(a) R is false, so both cannot be true.
(b) R is false, so this fails.
(c) Right: A is true, R is false.
(d) A is true, so this fails.

Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 1995, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.