What type of lens is primarily used in a goniolens for examining angle structures?

A goniolens is specifically designed for examining the drainage angle structures of the eye, typically used during gonioscopy. The primary characteristic of a goniolens is its ability to be placed directly on the cornea, allowing for a close-up view of the angle structures without significant distortion. This is why a contact lens is the correct answer, as it facilitates direct contact with the eye, providing a stable interface for the examination.

Contact lenses used in gonioscopy have features that aid in the visualization of the angle, such as a wide viewing area and a design that minimizes distortion of the structures being observed. While convex and concave lenses serve specific optical functions in other settings, they do not specifically address the need for direct and detailed examination of angle structures. Similarly, a prism lens is typically used for different applications, such as assessing diplopia or redirecting light, rather than for examining the anatomical angles of the eye.