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