The curve that occurs between the base curve and the peripheral curve is called

• A. tertiary/peripheral curve
• B. secondary/intermediate curve
• C. central curve
• D. primary curve

Answer: (B)

In a gas-permeable lens, the central optical region is the base curve, and the outer edge has its own curvature—the peripheral curve. The curve that sits between these two is the secondary curve, also called the intermediate curve in some terminology. This intermediate zone provides a smooth transition from the steeper base curve to the edge curve, helping the lens match the corneal and lid contours, support tear film flow, and promote stable fit and comfortable movement. The central curve is the base curve, and the edge-facing curve is the peripheral curve, so the transition between them is the secondary (or intermediate) curve.