Because Configuration 2 will model the reflected path in the beam splitter, we need to change Surface 4's Material from N-BK7 to MIRROR. Insert to the Multi-configuration Editor a GLSS operand for Surface 4 and specify a value of MIRROR for Configuration 2. A beam splitter or beamsplitter is an optical device that splits a beam of light into a transmitted and a reflected beam. It is a crucial part of many optical experimental and measurement systems, such as interferometers, also finding widespread application in fibre optic telecommunications.