where A itself is to be regarded as a vector.
We have next to consider the way in which the disturbance changes from
point to point. The most important case is that of plane waves with
constant amplitude A. Here f is the same at all points of a plane
("wave-front") of a definite direction, but changes as a linear
function as we pass from one such wave-front to the next. The axis of
x being drawn at right angles to the wave-fronts, we may write f = f0
- kx, where f0 and k are constants, so that (4) becomes