The refractory power of eye is normally 59 diopters. Two-thirds of this is contributed by

If all the refractive surfaces of the eye are algebraically added together and then considered to be one single lens, the optics of the normal eye may be simplified and represented schematically as a "reduced eye." This representation is useful in simple calculations. In the reduced eye, a single refractive surface is considered to exist, with its central point 17 millimeters in front of the retina and a total refractive power of 59 diopters when the lens is accommodated for distant vision. About two-thirds of the 59 diopters of refractive power of the eye is provided by the anterior surface of the cornea (not by the eye lens). The principal reason for this phenomenon is that the refractive index of the cornea is markedly different from that of air, (The refractive index of air is 1.00), whereas the refractive index of the eye lens is not greatly different from the indices of the aqueous humor and vitreous humor.Ref: Guyton and Hall textbook of medical physiology, 13th edition