Refraction at a curved surface(Refraction by lenses When parallel rays of light strike a spherical surface,each individual ray will be bent to a different degree and the rays may then all meet at a focus. The distance of this focus from the surface depends on 1-curvature of the surface 2-optical density of the two media concerned, 3-wavelength of the light Lenses are of many varieties,and Converging lens the ones most commonly used in ophthalmic practice are spherical and cylindrical with either convex or concave surfaces or combinations of these. Diverging lens 5Refraction at a curved surface (Refraction by lenses ) When parallel rays of light strike a sphericalsurface, each individual ray will be bent to a different degree and the rays may then all meet at a focus. The distance of this focusfrom the surface depends on 1- curvature of the surface 2- optical density of the two media concerned, 3- wavelength of the light 5 Lenses are of many varieties, and the ones most commonly used in ophthalmic practice are spherical and cylindrical with either convex or concave surfaces or combinations of these