Thicker on the edges than in the center; they cause parallel light rays emanating from an object to diverge.
Lens with one flat side and one concave side (curving inward).
Lens with one side curving inward; the greater the curvature, the more the light rays diverge.
Lens with both sides curving inward.
Lens where the concave side (curving inward) is more pronounced than the convex side (bulging outward).