Question

A light-year is a unit of:

• A. Time
• B. Light
• C. Intensity of light
• D. Distance

Hint:

Multiply the speed of light by one year and check what kind of quantity you get.

Answer 1

The Correct Option is D

Step 1: Recall the definition.
A light-year is defined as the distance travelled by light in vacuum in one year.
Step 2: Do the basic calculation.
Light travels at a speed of about \(3 \times 10^{8}\) m/s. Multiplying this speed by the number of seconds in one year gives a fixed length, roughly \(9.46 \times 10^{15}\) m.
Step 3: Identify the type of quantity.
Since the result of speed multiplied by time is a length, a light-year measures distance, not time or brightness.
Step 4: Rule out other options.
Time is wrong because although the word "year" appears in the name, the quantity being measured is how far light travels, not how long it takes. Light and intensity of light are not measured in light-years at all.
Final Answer: D) Distance.