Wave Profile of Harmonic Wave

Theorem
Let $\phi$ be a harmonic wave which is propagated along the $x$-axis in the positive direction with constant velocity $c$.

Then the wave profile of $\phi$ can be expressed as:
 * $\paren {\map \phi x}_{t \mathop = 0} = a \cos \omega x$

Proof
By definition, a harmonic wave is a wave whose wave profile can be expressed as a sine curve.

By definition, a sine curve can be expressed in the form:
 * $\map \phi x = a \map \sin {\omega x + \theta}$

where $a$, $\omega$ and $\theta$ are arbitrary constants.

We select $\theta$ so as to set:
 * $\theta = \dfrac \pi 2$

We then have:

Hence at time $t = 0$ the equation expressing a wave in the form of a sine curve is:
 * $\paren {\map \phi x}_{t \mathop = 0} = a \cos \omega x$

Hence the result by definition of wave profile.