Quantum Optics...wave amplitude question

Master J

In quantum optics, I've been looking at how one can show electromagnetic field is equivalent to a harmonic oscillator.

They give the E field as, in one dimension:

E_x (x,t) = A . cos(wt) . sin(kz) where the symbols have their usual meanings.

A is given as SQRT[2w^2 / Ve]

here, w is omega, V volume of cavity, e is permittivity of free space. I cannot find an answer however to where this amplitude A comes from. In two quantum optics books I have looked at, they simply introduce it without explanation.

Any help??