The anterior surface of the cornea, more Specifically the air-tear film interface, is responsible for 60% to 70% of the optical power of the eye; the other contribution comes from the crystalline lens. Because the shape of the cornea so profoundly affects the refractive state of the eye, many forms of refractive surgery attempt to alter its convex curvature. Myopia can be treated by increasing the central radius of curvature, making the cornea flatter with less optical power. Hyperopia treatment would be just the opposite, decreasing the radius of curvature to increase the optical power. Notable exceptions are various implants that have a higher index of refraction than does the corneal stroma (intrastromal implant) or water (intraocular lens implant). These implants rely on their intrinsic curvature to determine refractive correction.