 A mathematical representation of Huygens' principle can be used to start an equation.Consider a monochromatic complex plane wave formula_36 of wavelength λ incident on a slit of width "a".If the slit lies in the x′-y′ plane, with its center at the origin, then it can be assumed that diffraction generates a complex wave ψ, traveling radially in the r direction away from the slit, and this is given by