A lens is a converging glass plate whose faces are spherical, concave, or convex and almost parallel at the center. Let us consider a beam of parallel light rays (collimated) arriving at the first lens face. If these rays converge to a point (focus) after being refracted by the lens, the lens is converging or positive. If the rays diverge, the lens is diverging or negative (see Figure 2.1).
In the most common case, when the medium surrounding the lens is less dense (smaller index of refraction) than the lens material, a lens thicker at the center than at the edge is convergent, and a lens thinner at the center is negative. A thin lens may have different shapes, as shown in Figure 2.2 ...
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