Fresnel Biprism Equation Derviation- please help!! I recently did the Fresnel Biprism Experiment in order to calculate the wavelength of sodium light. As part of my write up I am to show the derviation of the formula d = (d1 * d2)^1/2 where d = actual separation of the virtual slits. Basically I dont have any idea how to derive this, all I know that it is supposed to come from the simple lens formula......does anyone have any idea how to derive this?? Any help would be greatly appreciated!!