What is the difference between refraction and rarefaction?

A beam of light passing from one transparent medium to another obliquely, undergoes an abupt change in direction . This bending of light ray, at the surface of separation of two media is called refraction.

Longitudinal waves like sound waves constitute of compressions and rarefractions.The region in a longitudinal wave where the particles are furthest apart is called a rarefraction.

It is the positions of particles of minimum density or pressure in the medium.