What is the name of the formula used to calculate induced prismatic effect caused by spectacle lenses?

The formula used to calculate the induced prismatic effect caused by spectacle lenses is known as Prentice's Rule. This rule provides a straightforward method to determine the amount of prism effect that arises when light passes through lenses that are decentered from the optical axis.

Prentice's Rule states that the prismatic effect (measured in prism diopters) can be calculated by multiplying the power of the lens (in diopters) by the decentration distance (in centimeters) from the optical center of the lens. This calculation is essential for opticians when fitting glasses, as any misalignment can lead to unwanted visual disturbances such as double vision.

In contrast, Snell's law describes how light refracts as it passes through different media and does not directly relate to the calculation of prism effects from lens decentration. Fresnel's equations deal with the reflection and transmission of light through surfaces but are not specific to lens-induced prismatic effects. Marginal deviation law pertains to the overall bending of light at the lens edges based on geometrical optics but does not provide the specific relationship needed for calculating induced prism like Prentice's Rule does.