What shape can the sides of a lens have?

The sides of a lens can have concave or convex shapes, which play crucial roles in how the lens focuses light. A concave lens is thinner at the center and thicker at the edges, causing it to diverge light rays that pass through it. Conversely, a convex lens is thicker at the center and thinner at the edges, which converges light rays to a focal point.

Understanding the shapes and their effects is fundamental in optics, as different combinations of concave and convex surfaces can create lenses with desired focusing properties for various applications like glasses, cameras, and microscopes. Flat surfaces are not suitable for focusing light effectively, as they do not curtail or converge the rays. Circular and rectangular shapes are not typical descriptors for lens functionality; rather, it's the curvature (concave or convex) that dictates how light interacts with the lens.