Question:

Light year is a unit of

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Speed multiplied by time gives a length, so ask what light covers in one year.
Updated On: Jul 17, 2026
  • Distance
  • Time
  • Sound
  • Light intensity
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The Correct Option is A

Solution and Explanation

Step 1: Understanding the Question:
The question asks what physical quantity a light year measures. The word "year" in the name makes many candidates pick time, so read the definition carefully.

Step 2: Key Formula or Approach:
A light year is defined as the distance light travels through vacuum in one year. Use \( \text{distance} = \text{speed} \times \text{time} \) to see which quantity comes out.

Step 3: Detailed Explanation:
The speed of light in vacuum is about \( 3 \times 10^{8} \) metres per second, and one year is about \( 3.15 \times 10^{7} \) seconds. Multiplying gives
\[ 3 \times 10^{8} \times 3.15 \times 10^{7} \approx 9.46 \times 10^{15}\ \text{m} \]
That is about 9.46 trillion kilometres, which is a length, not a duration. Speed multiplied by time always gives distance, so the unit measures distance. Astronomers use it because the gaps between stars are far too large to state in kilometres, so the nearest star Proxima Centauri is about 4.24 light years away.
Why the other options fail:
Time is the trap, planted by the word year in the name, but the year in the definition is only the time interval used to fix the length.
Sound has nothing to do with light travel, and sound is measured in decibels for loudness.
Light intensity is measured in candela or in lux, which are different units altogether.

Step 4: Final Answer:
A light year is a unit of distance, so option (A) is correct.
\[ \boxed{\text{Distance}} \]
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