Period of Harmonic Wave

Theorem
Let $\phi$ be a harmonic wave expressed as:
 * $\forall x, t \in \R: \map \phi {x, t} = a \map \cos {\omega \paren {x - c t} }$

The period $\tau$ of $\phi$ can be expressed as:
 * $\tau = \dfrac \lambda c$

where $\lambda$ is the wavelength of $\phi$.