y(x, t) = A cos(kx − ωt)

\frac{∂^{2}y}{∂x^{2}} = −k^{2} A cos(kx − ωt)

\frac{∂^{2}y}{∂t^{2}} = −ω^{2} A cos(kx − ωt)

k^{2} A cos(kx − ωt) = (\frac{k^{2}}{ω^{2}} ) ω^{2} A cos(kx − ωt)

\frac{k}{ω} = \frac{2π/λ}{2πf} = \frac{1}{fλ}

\frac{∂^{2}y}{∂x^{2}} = \frac{1}{V^{2}} \frac{∂^{2}y}{∂t^{2}}

\frac{∂^{2}E}{∂x^{2}} = \frac{1}{c^{2}} \frac{∂^{2}E}{∂t^{2}}

∇^{2}E = \frac{1}{c^{2}} \frac{∂^{2}E}{∂t^{2}}