What formula is used for measuring prism power on lenses at any point?

The formula used to measure prism power on lenses at any point is Prentice's rule. This rule specifically addresses how the displacement caused by prism is related to the amount of prism power, the distance from the optical center of the lens, and the angle at which light enters the lens.

Prentice's rule states that the amount of vertical or horizontal displacement (measured in millimeters) is equal to the prism power (measured in prism diopters) multiplied by the distance from the optical center in centimeters. This makes it an effective tool for understanding how much a lens will deviate light depending on where the light enters the lens, which is crucial in the field of optics.

The other choices contribute to optics in various ways, but they do not specifically pertain to measuring prism power on lenses at any point. For instance, Snell's law describes the relationship between angles of incidence and refraction when light passes between different media, while the Fresnel equations relate to the behavior of light when it reflects and refracts at interfaces. The lens maker's formula, meanwhile, is used to calculate the focal length of a lens based on its curvature and the refractive index of the material.