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If both surfaces are convex the lens is thicker at its center than its edges. A lens with one surface convex and one concave is called meniscus. Some lenses change the direction of the rays enough to cause the rays in a bundle to emerge convergent, that is, converging toward a common point. Whatever the mix of surfaces, if the lens is thicker at its center than its edges it is called a converging lens (having positive focal length). If it is thinner at its center than its edges it is called diverging (having negative focal length).
