What is measured by the amount that a prism displaces an image?

The term that accurately describes what is measured by the amount that a prism displaces an image is the "Prismatic Effect." This effect is expressed in units called prism diopters, which indicate the degree to which the prism shifts the position of an image relative to its original position.

When light passes through a prism, it is refracted, causing the light to bend and appear displaced. The extent of this displacement is quantitatively measured, and it helps opticians understand how much optical deviation a prism provides. This concept is crucial when prescribing prismatic corrections for patients, as it directly impacts vision correction strategies used in glasses.

In contexts related to vision and optics, "optical power" pertains to the ability of a lens to converge or diverge light, while "centrad" is a unit of angular measurement referring to a specific angle of incidence. The "diopter" is a unit that measures the optical power of lenses but not specifically the displacement caused by a prism. Understanding these distinctions highlights why "Prismatic Effect" is the correct concept associated with image displacement caused by prisms.