Question

Light year is the unit of

• A. time
• B. distance
• C. speed of light
• D. none of these

Answer 1

The correct option is B

distance

Definition of light year: A light year is defined as the distance travelled by light in vacuum in one year.

Determine the value of $1$ light year:

1. The speed of light in vacuum, $c=3\times {10}^8{\text{ms}}^{-1}$
2. Time passed, $T=365\text{days}$
3. Applying speed-distance-time formula: The distance travelled by light in time $T$ is:
4. $$\begin{gathered} S=c\times T \\ =\left(3\times {10}^8{\text{ms}}^{-1}\right)\times 365\text{days} \\ =\left(3\times {10}^8{\text{ms}}^{-1}\right)\times \left(365\times 24\times 3600\text{s}\right) \\ =9.46\times {10}^{15}\text{m} \end{gathered}$$
5. Therefore, light year is a unit of distance.

Hence, the correct option is (b).