The thickness of an aluminum foil is measured using 315-nm wavelength light in a Michelson interferometer that has its movable mirror mounted on a very finely threaded distance gauge (commonly called a "micrometer screw gauge" or often just a "micrometer"). There is a difference of 27 fringes in the observed interference pattern when the micrometer clamps down on the foil compared to when the micrometer is empty (clamped down on nothing). Calculate the thickness of the foil?