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Piane wave terminology

In the above equation the first factor changes amplitudę, the second changes phase. The

directions of maximum rates of change of amplitudę and phase are a and 3 respectively.

The piane perpendicular to 3 is a piane of constant phase; that such planes exist is why

Solutions of this type are called piane waves. The piane perpendicular to a is a piane of

constant amplitudę. If such variations of amplitudę exist, i.e. if a 6= 0, the wave is called

a non-uniform piane wave. If no such variation of amplitudę exists, i.e. if a = 0, the wave

is called a uniform piane wave.