A nanometer (nm) is a unit of length in the metric system equal to one billionth of a meter. It's often used to measure microscopic things like atoms and molecules.

A light-year is a unit of astronomical distance, representing the distance that light travels in vacuum in one Julian year. It's a very large distance, used for measuring distances between stars and galaxies.

This is because a nanometer is incredibly tiny, while a light-year is astronomically vast. The number of nanometers in a light-year is enormous, so 100,000,000 nanometers is a minuscule fraction of a single light-year.

Direct practical uses are rare due to the extreme difference in scale. However, it's valuable for educational purposes and in theoretical science to illustrate scale differences or in hypothetical cosmic models.