Review: A thin film of MgF$2($n$=1\mathrm{.}38)$ is applied to a flat piece of polycarbonate $($n$=1\mathrm{.}586)$ to prevent glare.The light is perpendicular to the surface $($so you may ignore refraction$).$A$.$ What is the speed of the light as it passes through the MgF$2$ and the polycarbonate?B$.$ What are the thinnest $2$ thicknesses of MgF$2$ that will produce destructive interference for awavelength of $550$ nm$?$