How many curves does a bi-toric lens have?

A bi-toric lens is specifically designed to correct astigmatism, which involves the presence of both spherical and cylindrical powers in the lens. These lenses accommodate two different meridians of the eye's surface, which is why they have distinct curvature profiles.

The correct answer highlights that a bi-toric lens has one spherical curve and one cylindrical curve. The spherical curve is responsible for addressing any spherical refractive error (myopia or hyperopia), while the cylindrical curve corrects the astigmatic component. This combination is essential, as astigmatism involves varying refraction in different directions.

In contrast, the other answer choices do not accurately describe the bi-toric lens. For instance, stating that there are two spherical curves would imply that there is no correction for astigmatism, which is not the case. Similarly, two spherical and cylindrical curves imply a configuration that does not align with the standard characterization of a bi-toric lens, which specifically has one of each power type. Therefore, the clarity provided by the choice that cites one spherical and one cylindrical curve aptly conveys the essential features of a bi-toric lens.