How is a convex lens characterized in terms of its shape?

A convex lens is characterized by being thicker at the center than at the edges. This shape allows the lens to converge light rays that are parallel to its principal axis. When light hits the convex lens, it refracts, bending towards the center, which facilitates the focusing of light to a point known as the focal point. This property makes convex lenses essential in various optical devices, such as cameras, eyeglasses for farsightedness, and magnifying glasses.

The other options describe different lens types. For instance, a lens that is thinner at the center than at the edges would be a concave lens, which diverges light rays instead of converging them. A lens that is of equal thickness across its diameter does not allow for any significant bending of light, making it ineffective in focusing light. Lastly, a lens that is flat across its diameter does not provide the curvature necessary for refraction, resulting in no focusing effect. Thus, the unique shape of a convex lens is crucial for its ability to gather and focus light effectively.