At a given instant of time the ripples on the surface of the pond can be described using the expression:

psi-z = Re{e^i*(kx(x) + ky(y))}

where kx and ky are constants (Note: the x is subscript of k)

and k-x = 2pi/lambda-x

Derive an equation involving x and y that defines the crest that passes through the origin of the coordinate system