A ray of light propagating in an isotropic medium with refractive index n varying gradually from point to point has a curvature radius p determined by the formula 1/p = ∂/∂N(In n). Where the derivative is taken with respect to the principal normal to the ray derive this formula, assuming that in such a medium the law of refraction n sin 0 = const holds. Here 0 is the angle between the ray and the direction of the vector Vn at a given point.