What is an oculus?

• A. A circular window.
• B. The circular opening at the apex of a dome.
• C. A circular lens used in cameras.
• D. A circular decorative motif.

Answer: (B)

An oculus is the circular opening at the apex of a dome. This architectural feature provides light and a sense of openness to the space below, often acting as a skylight in classical buildings like the Pantheon. It’s defined by its location—at the top of the dome—and its function as an opening, not a wall window or a decorative element. While a circular window shares the same shape, it sits in a wall rather than at the crown of a dome. A circular lens is something you’d find in photography, not in architecture, and a circular decorative motif is ornament, not an opening.