What does the interval of Sturm refer to?

The interval of Sturm specifically refers to the distance between two focal planes in the context of optics. When light passes through a lens, it is focused at a particular point known as the focal point. However, for certain lenses—particularly astigmatic lenses—there can be two distinct focal points along the same axis due to variations in curvature on the lens surface. The interval of Sturm is the measurement between these two points and is relevant in the diagnosis and correction of astigmatism, helping to optimize vision correction for individuals affected by this condition.

Understanding this term is crucial in optics as it directly impacts how lenses are designed and prescribed, ensuring that patients receive the best possible correction for their vision needs. This distinction makes it clear why this choice is the correct answer.