Two thin parallel slits are made in an opaque sheet of film. When a monochromatic beam of ligbt is shone through them at normal incidence, the first bright fringes in the transmitted light occur in air at ± 18.0" with the original direction of the light beam on a distant screen when the apparatus is in air. When the apparatus is immersed in a liquid, the same bright fringes now occur at ± 12.6°. Find the index of refraction of the liquid.