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Q.
Light year is used to measure
Delhi UMET/DPMTDelhi UMET/DPMT 2007Physical World, Units and Measurements
Solution:
Light year is important unit of long distances
(to measure distance of stars, terrestrial objects).
One light year is the distance travelled by light in vacuum in one year.
As velocity of light in vacuum is
$3 \times 10^{8} ms ^{-1}$ and $1$ year $=365 \times 24 \times 60 \times 60$ second,
therefore, $1$ light year
$=3 \times 10^{8} \times(365 \times 24 \times 60 \times 60)$ metre
$\therefore 1$ light year $=9.46 \times 10^{15} m$