Are there ways to change the wavelength of an EM wave while still preserving all geometry of the wave (for instance, preserving an interference pattern which has, of course, irregular geometry in between the minima and maxima of the wave)? I've heard some crystals can change the wavelength but I'm thinking that perhaps some geometry would be lost due to parts of the wave running into the molecules and/or atoms that the crystal is made of. Would the wavelength change be a coherency-preserving process? Is an electron wave the same as an EM wave (besides having a higher frequency)? And if no, is there a way to change an electron wave's wavelength while preserving geometry and coherency as described above? Thank you!