The intersection of the two rays marks the position of the image. To check the accuracy of your drawing, draw a third ray through the center of the lens. This ray is not bent. It should pass through the intersection of the other rays that you have drawn. Diverging lenses only form virtual images.
R is negative, if x o and the center curvature are on the same side of the lens.
R 1 is the radius of the surface closest to x o . Many optical instruments have more than one optical element. A combination of mirrors and lenses can be analyzed by treating the image of the first element as the object of the second element, and so on. If the image of the first element falls behind the second element we can still use the lens or mirror equation, but we then must use a negative object distance x o . A contact lens is made of plastic with an index of refraction of 1.5.