The part of a longitudinal wave where the particles of the medium are far apart is called a rarefaction.

Here's why:

* Longitudinal waves: In these waves, the particles of the medium vibrate parallel to the direction the wave travels.

* Compression: A compression is a region where the particles are close together.

* Rarefaction: A rarefaction is a region where the particles are spread out or farther apart.

Think of a slinky:

* When you push the coils together, you create a compression.

* When you pull the coils apart, you create a rarefaction.

These compressions and rarefactions alternate as the longitudinal wave travels through the medium.