The correct answer is option 1: very large distances. A light year is a unit of measurement used to describe astronomical distances. It represents the distance that light travels in one year in a vacuum. Since light travels at a speed of approximately 299,792 kilometers per second, or about 186,282 miles per second, in one year it can cover an immense distance. Therefore, a light year is used to quantify these vast distances in space.

Option 2, time intervals in years, is not correct because a light year is not a measurement of time, but rather a measurement of distance. It does not represent a specific duration of time.

Option 3, the amount of light received on Earth in a year, is also not correct. While the name "light year" may suggest that it is a measure of the amount of light received on Earth in a year, it actually refers to the distance light travels, not the amount of light.

Option 4, mass of atoms, is unrelated to a light year. The mass of atoms is typically measured in units such as grams or kilograms, and has no direct connection to the concept of a light year.

In conclusion, a light year is a unit of measurement used to quantify very large distances in space.