Question

A light year is a unit of
A. distance
B. time
C. Speed
D. Mass

Answer 1

Hint: We know the distance travelled by light in one year is called light year(ly). It is used to measure large distances. A light year is also related to astronomy and parsec. These units are also used to measure very large distances.

Complete step by step answer:
We know that one light year is equal to the distance that light travels in a year.
$ 1ly = c \times 1yr$
Here c is the speed of light in vacuum which is equal to $3 \times {10^8}\;\dfrac{m}{s}$ and 1 ly means one light year. Hence on simplifying the above relation, we get,
\[ \Rightarrow 1ly = 3 \times {10^8}\;m/s \times 365 \times 24 \times 60 \times 60s\].
\[ \Rightarrow 1ly = 9.46 \times {10^{15}}{\rm{m}}\]
Relation between light year (ly) and astronomical unit (A.U)
$1({\rm{ly) = 9}}{\rm{.46}} \times {\rm{1}}{{\rm{0}}^{15}}m$ …… (I) and,
${\rm{1A}}{\rm{.U = 1}}{\rm{.5}} \times {\rm{1}}{{\rm{0}}^{11}}m$ …… (II)
Here 1 AU means 1 astronomical unit.
On dividing equations(I) and (II), we get:
$\dfrac{{1{\rm{ly}}}}{{1{\rm{A}}{\rm{.U}}}} = \dfrac{{9.456 \times {{10}^{15}}m}}{{1.5 \times {{10}^{11}}m}}$
$ \Rightarrow \dfrac{{1{\rm{ly}}}}{{1{\rm{A}}{\rm{.U}}}} = 6.3 \times {10^4}$
$\therefore 1{\rm{ly = 6}}{\rm{.3}} \times {\rm{1}}{{\rm{0}}^4}{\rm{A}}{\rm{.U}}$
Relation between the light year (ly) and parsec
Again $1{\rm{ly = 9}}{\rm{.46}} \times {\rm{1}}{{\rm{0}}^{15}}m$ …… (III)
And ${\rm{1parsec = 3}}{\rm{.1}} \times {\rm{1}}{{\rm{0}}^{16}}m$ …… (IV)
On dividing equations (III) and (IV), we get
$\dfrac{{1{\rm{parsec}}}}{{1{\rm{ly}}}} = \dfrac{{3.1 \times {{10}^{16}}m}}{{9.46 \times {{10}^{15}}m}}$
$\dfrac{{1{\rm{parsec}}}}{{1{\rm{ly}}}} = 3.28$
$\therefore 1{\rm{parsec = 3}}{\rm{.28ly}}$

Light year is a unit to measure distance. Hence, option (A) is correct.

Additional information:
The light year is used to measure distances in space because the distances are so big that a large unit of distance is required for measuring. Using a light-year as a distance measurement unit has another advantage — it helps you identify the age. Let us consider a star that’s 1 million light-years away. The light from the star has travelled at the speed of light to reach us. Therefore, the star’s light took 1 million years to get to us, and the light that we are seeing was illuminated a million years ago. So the star that we see is really how the star looked a million years ago, not how it looks today.

Note:
In the same way, our sun is approximately 8 light minutes away. If the sun were to suddenly explode immediately, we wouldn’t realize about it for eight minutes because that is how long it might deem the sunshine of the explosion to get here.