Convex-concave lenses, also know as meniscus lens due to its shape, can be either positive or negative depending on the relative radius of the two surfaces. A negative meniscus lens has a steeper concave surface and is thinner at the centre than at the periphery. Conversely, a positive meniscus lens has a steeper convex surface and is thicker at the centre than at the periphery. An ideal thin lens with two surfaces of equal curvature would have zero optical power, means that it would neither converge nor diverge light.However,all the real lens has thickness somehow, which makes a real lens with identical curved surfaces slightly positive. In order to obtain exactly zero optical power, a meniscus lens must have slightly unequal curvatures to account for the effect of the lens thickness.
Optical Windows
