Question

Assertion (A):
Light year is the unit of time.
Reason $(R)$:
Light year is the distance traveled by light in a vacuum in one year.

a. $A$ and $R$, both are correct and R is the correct explanation of $A$.
b. $A$ and $R$, both are correct and R is correct not correct explanation $A$
c. $A$ is true and $R$ is false
d. $A$ is false and $R$ is true.

Answer 1

Hint: An assertion is given and the reason is also given. In such kinds of questions, we have to read both the assertion and the reason carefully and check whether both are true and if the reason is the correct explanation for the given assertion.

Complete step by step answer:
A light-year is an astronomical unit. It is used to express the distance from celestial objects. A light-year is a distance traveled by the light in one year through the vacuum. It is mostly used for expressing the distances from stars and other celestial objects.

The assertion is that light-year is the unit of time. We know that light-year is not the unit of time but the unit of distance. Hence we know that the assertion is not correct. The reason is given as light-year is the distance traveled by light in a vacuum in one year.

Hence, the correct answer is option (D).

Additional Information:
By definition, a light-year is the product of one Julian year and the speed of light. Another astronomical unit for measuring distance in astronomy is parsec. Distances between stars, the distance between planets and stars, etc. are expressed in terms of astronomical units.

Note: There is a probability of getting confused about a light-year being the unit of time due to the word ‘year’ in it. But it does not have any relation with time. It is the unit of distance. Students have to remember this carefully.