What is a cylinder in lens terminology?

In lens terminology, a cylinder refers to a surface with no curvature in one meridian. This concept is fundamental in the design and function of cylindrical lenses, which are specifically created to correct astigmatism. Astigmatism is caused by an irregular curvature of the cornea or lens in the eye, leading to blurred vision. The cylindrical portion of a lens compensates for this irregularity by having a flat surface in one meridian and a curved surface in another, allowing it to bend light differently in those two directions.

This characteristic is crucial for creating the visual clarity necessary for individuals with astigmatism, as it helps focus light onto the retina more effectively than a standard spherical lens. By having one meridian without curvature, cylindrical lenses operate by correcting the unequal refractive power in different axes of vision. This is why understanding the properties of cylinders in optics is essential for providing precise corrections to meet individual vision needs.