In order for diffraction to occur, the slit width must be on the order of the wavelength, correct? I'm puzzled because if the wave is measured along the x-axis while the slit is along the y-axis, I don't see the connection. Is this best described as a quantum mechanical effect? By passing through the slit the position of the wave is well known, resulting in a great uncertainty in the momentum which leads to the "spreading out" of the wave. What's the classical analong? Hyugens' principle?