A narrow stream of electrons with kinetic energy \(T\) \(=10 \mathrm{keV}\) passes through a polycrystalline aluminium foil, forming a systeun of diffraction fringes on a screen. Calculate the interplanar distance corresponding to the reflection of third order from a certain system of crystal planes if it is responsible for a diffraction ring of diameter \(D=3.20 \mathrm{~cm} .\) The distance between the foil and the screen is \(l=10.0 \mathrm{~cm}\)