In basic physics and astronomy, a light year is a commonly used unit related to which physical quantity when describing distances in the universe?

Introduction / Context:
This question is from physics and astronomy, focusing on commonly used units. The term light year sounds like it might measure time, but in scientific usage it actually measures distance. Many exams test whether you can correctly identify what a light year represents when talking about stars and galaxies.

Given Data / Assumptions:

• The question asks what physical quantity a light year is related to.
• The options are distance, acceleration, velocity, and intensity of light.
• We assume the standard definition used in astronomy and physics.

Concept / Approach:
A light year is defined as the distance that light travels in vacuum in one year. Since light has a very high speed, this distance is enormous, making the light year a convenient unit for expressing distances between stars and galaxies. Therefore, even though the word “year” appears in the term, it is a unit of distance, not time.

Step-by-Step Solution:
Step 1: Recall the definition of the speed of light, which is approximately 3 * 10^8 metres per second.Step 2: Understand that a light year multiplies this speed by the number of seconds in one year to obtain a distance.Step 3: Recognise that this result is a very large number of metres or kilometres, making it practical to treat one light year as a large distance unit.Step 4: Compare the options and see that only “distance” matches this interpretation.Step 5: Select distance as the correct physical quantity.

Verification / Alternative check:
Any basic physics or astronomy textbook will state that a light year is a unit of distance equal to the distance light travels in one year. Tables of scientific units and exam revision notes also list it under distance units. This consistent presentation removes any doubt about its meaning.

Why Other Options Are Wrong:
Acceleration: Measured in units like metre per second squared, not in light years.Velocity: Measured in metre per second or kilometre per hour, not in light years.Intensity of light: Related to luminous flux or power per unit area and has its own units, not light years.

Common Pitfalls:
The presence of the word “year” often misleads students into thinking it is a measure of time. Remember that the term combines speed (of light) and time (one year) to produce a distance. When you see “light year”, always think of very large distances between celestial objects rather than any time interval.

Final Answer:
A light year is related to the physical quantity of distance.