2021 GS Paper I, all questions

UPSC Prelims 2021 · GS Paper I

Question 76: Science and Technology

Which one of the following is a reason why astronomical distances are measured in light-years?

aDistances among stellar bodies do not change.
bGravity of stellar bodies does not change.
cLight always travels in straight line.
dSpeed of light is always same.

AnswerUPSC’s answer is (d).

A light-year is the distance light travels in one year, about 9.46 trillion kilometres. Astronomers use it because the speed of light in vacuum is constant, so the unit is a fixed, reliable yardstick for the huge distances between stars. Distances among stellar bodies do change as they move, the gravity of bodies varies with mass, and light bends in strong gravitational fields, so options (a) to (c) are false.

Why each option is right or wrong
  • (a) Wrong: stars and galaxies move, so distances change.
  • (b) Wrong: gravity differs from body to body.
  • (c) Wrong: light can bend in strong gravity, so this is not a valid reason.
  • (d) Right: the speed of light in vacuum is constant, so the unit is a fixed standard.

Answer: UPSC’s key. Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2021, General Studies Paper I, Series A. The paper on upsc.gov.in.