What is the difference between refraction and rarefaction?

What is the difference between refraction and rarefaction?

Refraction occurs when a wave crosses a boundary from one medium to another. A wave entering from an angle will change direction

Rarefraction refers to the "bending of wavelengths around an edge"of an object. diffraction depends on the size of the object relative to the wavelength of the waves.

The  grandmother cell  is a hypothetical  neuronthat represents a complex but specific concept or object.[1]  it activates when a person "sees, hears, or otherwise sensibly discriminates"[2]  a specific entity, such as his or her grandmother. the term was in use at least as early as 1966 amongst staff and students in the department of experimental psychology, university of cambridge, england. a similar concept, that of the  gnostic neuron, was proposed two years later