To keep unwanted light from reflectng from the surface of eyeglasses or other lenses, a thin film of a material with an index of refracton n=1.38 is...

To keep unwanted light from reflectng from the surface of eyeglasses or other lenses, a thin film of a material with an index of refracton n=1.38 is coated onto he plastic lens (n=1.55). It is desired to have destructive interference for λ=550 nm because that is the center of the visible spectrum. What is the thinnest film that will do?