I just learnt that there is a well established theoretical picture that explains the change of momentum associated with direction change during a diffraction event, in terms of quantized momentum transfer to the diffracting object. For example, https://en.wikipedia.org/wiki/Duane's_hypothesis This picture is also supposed to be applicable to diffraction during transmission through a single slit or a double slit. I am just wondering what kind of forces actually mediate the exchange, e.g. in the cases of electron diffraction and photon diffraction. I found hints suggesting that this could be related to phonons within the screen material, but - if so - wouldn't the diffraction then be a function of the solid state physics happening inside the screen? This leads to my actual question : is it reasonable that the force that "mediates" the deflection and momentum exchange is a kind of Casimir effect occuring within the slit volume, where certain EM modes are suppressed by geometry? Also, what force would mediate the momentum transfer when a photon is diffracted through a slit?