In astronomy, a light-year is a unit used to describe how far light travels in one year. What physical quantity does this unit actually measure?

Introduction / Context:
A light-year is a very common term in astronomy and popular science writing. Many learners hear it in documentaries about galaxies, stars, and planets, but it is easy to confuse the word year with the idea of time. Competitive exams often test whether candidates know that a light-year is not a measure of time at all, but a measure of distance. Understanding this clearly helps you interpret astronomical scales correctly and prevents basic conceptual errors when reading scientific material about the universe.

Given Data / Assumptions:

• Light travels in a vacuum with a speed of about 3 * 10^8 metres per second.
• A year here means one standard Earth year used in scientific calculations.
• The term light-year is used mainly in astronomy to express very large distances.
• We are asked which physical quantity is represented by the unit light-year.

Concept / Approach:
In physics, every unit is tied to a specific physical quantity. For example, metre is a unit of length, kilogram is a unit of mass, and second is a unit of time. A light-year is defined as the distance that light travels in one year in vacuum. The word year appears only because we are multiplying the speed of light by a time period of one year to get a distance. Therefore, even though the name contains the word year, the underlying quantity is distance, not time or intensity or anything else.

Step-by-Step Solution:
Step 1: Recall that speed = distance / time, so distance = speed * time. Step 2: Take speed of light as c = 3 * 10^8 m/s for approximate calculation. Step 3: One year has nearly 365 days, so the time in seconds is about 365 * 24 * 60 * 60. Step 4: Multiply c by this number of seconds to obtain a very large number of metres. Step 5: This huge value is defined as one light-year, clearly representing a length or distance.

Verification / Alternative check:
You will never see light-years used to measure durations of events like a cricket match or the age of a child. Instead, they are used when describing how far away stars and galaxies are. Astronomers also sometimes use parsec as another unit of distance. All such usage in scientific articles and textbooks confirms that light-year is always treated as a distance unit that simplifies the representation of extremely large lengths in space.

Why Other Options Are Wrong:
Intensity of light is measured in units such as candela or watt per square metre, not in light-years, so option A is incorrect. Mass is measured in kilograms or grams, so option B does not match the definition. Time is measured in seconds, minutes, hours, or years, but the specific term light-year is not used for time, making option D incorrect. Only distance fits both the definition and real scientific usage of the term light-year.

Common Pitfalls:
A very common mistake is to focus only on the word year and conclude that a light-year is a time interval. Another error is to confuse it with speed, thinking that it means the speed of light itself, which is actually measured in metres per second. To avoid such confusion, always remember the formula distance = speed * time and recognise that light-year combines speed of light and one year of time to yield a distance. Keeping the physical quantity in mind will help you answer similar questions quickly in exams.

Final Answer:
The unit light-year is used in astronomy to measure very large distance, not time or intensity or mass.