Question

Light year is a measurement of
A. speed of aeroplanes
B. speed of light
C. stellar distances
D. speed of rockets

Answer 1

Hint: Light year is a unit of distance. By the name itself, we can understand that the unit light year is related to light. The value of one light year is equal to the distance that light (in vacuum) covers in one year

Formula used:
$d=vt$
where d is the distance, v is speed and t is time.

Complete step by step answer:
Let us first understand what is meant by light year.Light year is a unit of distance. By the name itself, we can understand that the unit light year is related to light. The value of one light year is equal to the distance that light (in vacuum) covers in one year. It is based on the formula that distance is equal to the product of speed and time i.e. $d=vt$, where d is the distance, v is speed and t is time.

We know that the speed of light in vacuum is equal to $c=3\times {{10}^{8}}m{{s}^{-1}}$.
The time spent in a year is equal to $t=365\times 24\times 60\times 60=31536000s$
Therefore, the distance covered by light travelling vacuum in one year is equal to
$d=ct\\
\Rightarrow d=3\times {{10}^{8}}\times 31536000 \\
\Rightarrow d = 94608000\times {{10}^{8}}m$.
$\therefore d=94608\times {{10}^{11}}m$
This means that 1 light year is equal to $94608\times {{10}^{11}}$ metres.Suppose an object is at a distance of $94608\times {{10}^{11}}$ metres, then we can say that it is one light year away. Now, we know that the light year is a unit length and it is a measurement of distance. It is used in measuring the distance of celestial bodies.

Hence, the correct option is C.

Note: You may be thinking why we use a unit of such large magnitude. One light year is a very large distance and cannot be used for the date to date measurements (since the distance on earth is not so large).However, it is very useful in writing down the distances of faraway celestial bodies like stars and planets. The distances of these celestial bodies are so big that even light takes years to travel these distances. That is why we use a unit of larger magnitude so the numerical value of the measurement is small.