Question

Which of the following is equal to one light year?
${\text{A}}{\text{.}}$ 3.26 parsec
${\text{B}}{\text{.}}$ 3.26 km
${\text{C}}{\text{.}}$ 3.26 A.U.
${\text{D}}{\text{.}}$ $\dfrac{1}{{3.26}}$ parsec

Answer 1

Hint: Here, we will proceed defining the term light year. Then, we will write down the requirement of this unit of measuring distance. Then, we will mention the relation between the unit parsec and light years.

Complete step by step answer:
A light-year is the distance one light beam travels in one year in a vacuum. Specifying a vacuum is critical, since light slows as it passes through any form of matter. (It travels about 25 percent slower in water , for example.) Most of the universe is a near-perfect vacuum, so astronomers can generally assume that light is moving at its highest speed.
As astronomers look at stars using their telescopes things are different. The distances are immense. For example, Earth's closest star (apart from our sun) is about 24,000,000,000,000 miles (38,000,000,000,000 kilometers) away. This is the proximity light. There are stars which are larger than that by billions of times. When you start talking about these kinds of distances, it's just not a realistic unit to use a mile or kilometer because the numbers get too high. Nobody wants to write or speak about numbers with 20 digits in them.
So people use a device called a light-year to calculate very long distances. Light flies at an altitude of 186,000 miles per second (300,000 km per second). Therefore, 186,000 miles (300,000 kilometres) is a light second.
One Astronomical unit (A.U.) is defined as the mean distance between the planet Earth and the sun. 1 A.U. is approximately equal to 150 million kms.
One parsec is defined as the distance to an object whose parallax angle is equal to one arcsecond.
A parsec is equal to 3.26 light years and because a light year the distance light travels 9.4 trillion km in one year, 1 parsec is equal to 30.8 trillion km.
i.e., 1 parsec = 3.26 light years
$ \Rightarrow $ 1 light year = $\dfrac{1}{{3.26}}$ parsec

Therefore, option D is correct.

Note:
Age can be measured using a light year. Let's presume a star is only one million light years ahead. At the speed of light the light from that star has moved to reach us. Hence it took the light of the star 1 million years to get here, and the light we see was created 1 million years ago. And the star we see is just what the star appeared like a million years ago, not how it looks like today.