Base curve is located on which surface of a contact lens?

• A. Front surface
• B. Back surface
• C. Edge
• D. Inside

Answer: (B)

Base curve refers to the curvature of the lens surface that sits on the eye—the posterior, or back, surface. This is the surface that contacts the tear film and the cornea, so its curvature largely determines how the lens fits, centers, and interacts with the tear layer. A steeper back surface (smaller radius) tends to hug the cornea more tightly, while a flatter back surface (larger radius) sits more loosely. The front surface is the outer, optical face of the lens and affects vision and comfort, but it does not define how the lens sits on the eye. The edge is the peripheral rim and likewise does not set the fit against the cornea.