Which of the following statement/statements is/are true regarding the light year?

1.Light year is a unit of distance.

2.Light is a unit of time.

3.A light year is the distance that light travels in one Earth year.

4.Light year is the measurement of light intensity.

The correct option/answer is: Only 1 and 3 are correct.

Let us analyze each statement step-by-step to understand the concept of a light year:

Statement 1: Light year is a unit of distance.
This statement is correct. Despite having the word "year" in its name, a light year does not measure time. Instead, it is an astronomical unit used to measure the vast distances between stars, galaxies, and other celestial objects across the universe.

Statement 2: Light is a unit of time.
This statement is incorrect. Light is electromagnetic radiation that is visible to the human eye, not a unit of measurement for time (units of time include seconds, minutes, hours, years, etc.). Even if the statement intended to say "Light year is a unit of time", it would still be incorrect because, as established in Statement 1, a light year is a unit of distance, not time.

Statement 3: A light year is the distance that light travels in one Earth year.
This statement is correct. By definition, one light year is the distance that a beam of light travels through a vacuum in one Julian year (approximately 365.25 days).
Since the speed of light in a vacuum ($c$) is approximately $3\times {10}^8\text{m/s}$, we can calculate the distance of one light year as:
$\text{1 light year}=\text{speed of light}\times \text{time in one year}$
$\text{1 light year}\approx (3\times {10}^8\text{m/s})\times (365.25\times 24\times 60\times 60\text{s})$
$\text{1 light year}\approx 9.46\times {10}^{15}\text{meters}$ (or about 9.46 trillion kilometers).

Statement 4: Light year is the measurement of light intensity.
This statement is incorrect. Light intensity (or luminous intensity) is measured in candelas ($\text{cd}$) or lumens ($\text{lm}$), not light years.

Therefore, only statements 1 and 3 are correct.