Question

What is the definition of light year?

• A. The speed at which light travels for one earth year in space.
• B. The time taken by the light to reach Earth from the Sun.
• C. The distance which Earth travels in one earth year in space.
• D. The distance which light travels in one earth year in space.

Answer 1

The correct option is D The distance which light travels in one earth year in space.

A light year is defined as the distance travelled by light in one earth year in space.
1 light year ($\text{ly}$) $\approx 9.5\times {10}^{12}\text{km}$
It is used as a unit to express distances to celestial objects. E.g. if a star is $4\text{ly}$ away from earth, it means that light from the star will take $4$ earth years to reach us.