ABO NOCE Practice Test 2025 - Free Optician Certification Practice Questions and Study Guide

To understand why the choice of two base-up prisms (BU/BU) is the correct answer for canceling a prism effect, we need to delve into how prisms work and how they interact with light.

Prisms refract light and can bend it in specific directions depending on the base orientation. The base of a prism is the wider part of the triangle. By placing two prisms with the same base orientation together, such as two base-up prisms, their effects can counterbalance each other. This means that the light passing through both will ultimately have its deviation canceled out, returning close to its original path.

When two prisms with identical orientations are used, they exert their effects equally in opposite directions. Therefore, the angle of refraction introduced by the first prism is nullified by the second prism of the same orientation. In this case, the net effect of two base-up prisms results in a neutral effect, effectively canceling any prismatic deviation that might be introduced by the individual prisms.

In contrast, combinations involving prisms with different base orientations or different bases do not exhibit the same cancelling property. For instance, using a base inward (BI) and base outward (BO) would create a combined deviation rather than