The following formula, called the Lensmaker Equation, is used to determine whether a lens will behave as a converging or diverging lens based on the curvature of its faces and the relative indices of the lens material [n_{1}] and the surrounding medium [n_{2}].
Usually the expression
is treated as a constant (K_{shape} ) allowing us to work more often with the following second form of the equation:
Remember that K_{shape} represents the shape of the lens which remains constant regardless of the type of surrounding medium [n_{2}] into which the lens is used.
If this expression yields a negative value for 1/f, then the lens is diverging; a positive 1/f means that the lens is converging.
