Sound, for example …
Acoustic impedance, Z = ρv, is important when calculating the amount of reflection and transmission at an interface. For the reflected portion, notice that when Z2 < Z1, the sign of the reflected portion is negative, which indicates a change of phase.
|R/A0 = (Z2 - Z1)/(Z2 + Z1)||(reflected pressure amplitude)|
|T/A0 = 2Z2/(Z2 + Z1)||(transmitted pressure amplitude)|