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Home› UPSC CSE› Prelims GS Paper I› 2021 questions› Question 57

UPSC CSE Prelims

Subject: Physics Chapter: Astronomy And Space Science

2021 PYQ

Question 57

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

Choose one answer 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.

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Correct answer: D

Correct Option: D (Speed of light is always same.) Explanation: A light-year is a distance defined by the fixed speed of light in vacuum multiplied by one year, convenient for immense astronomical separations. Option-wise Analysis: A. Incorrect-Stars move, so distances between celestial bodies can change. B. Incorrect-Stellar gravitational fields are not constant or a basis for distance units. C. Incorrect-Light can be bent by gravity/refraction; its straight-line path is not the reason. D. Correct-the constant speed of light in vacuum defines a reproducible long-distance unit.

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