By definition an ophthalmic prism is wedge-shaped lens of glass or plastic that is thicker at one edge (the base) than at its opposite edge (the apex). Prisms usually are prescribed to correct faulty eye-muscle imbalances in which both eyes do not act as a team. In these cases, the refractive doctor prescribes the proper amount of prism and the position of the base in order to correct the affected muscles. Prisms are sometimes inadvertently introduced when prescription lenses are not centered properly in their eyeglass frames. These are called unwanted prisms. Therefore a quality oriented lab is required for centering accuracy.
For teaching purposes an ophthalmic prism is traditionally illustrated in the form of a pyramid or triangle that dramatizes the difference in thickness between its base and its apex. Prisms in ophthalmic lenses unless present in strong amounts are cosmetically inconspicuous.
Light rays entering and leaving a prism are bent toward the base of the prism and away from the apex. This causes objects to become displaced away from the base of the prism and toward its apex. Thus if an object is viewed through a base down prism, it will seem to become displaced upward. Similarly, objects appear to shift downward when viewed through a base-up prism, to the right or left when seen through a base-out or base–in prism, and so forth.