Why are all three lights on the beam splitter lit at the same time

All three lights appear on a beam splitter because the device simultaneously transmits, reflects, and can produce secondary reflections or wavelength-dependent outputs depending on its design.Basic Beam SplittingA beam splitter divides an incident light beam into two primary paths: transmitted and reflected. The proportion of light in each path is determined by the splitter's design, such as a 50/50 ratio for standard non-polarizing splitters . The transmitted beam passes through the splitter, while the reflected beam is redirected at a specific angle, often 90 degrees relative to the input .Secondary or Ghost BeamsIn plate-type beam splitters, a portion of light can reflect off the back surface of the glass, creating a faint ghost beam in addition to the main transmitted and reflected beams . This is often observed as a third light and is more noticeable in high-intensity or coherent light sources like lasers. Anti-reflection coatings or wedged substrates are used to minimize this effect.Polarization and Wavelength EffectsPolarizing beam splitters separate light based on polarization, producing two beams with orthogonal polarization states. Dichroic or multi-band beam splitters can split light into multiple beams based on wavelength, allowing three or more distinct outputs if the incident light contains multiple spectral components . In such cases, each “light” corresponds to a different polarization or wavelength channel.Practical ImplicationsSeeing three lights on a beam splitter is common in laboratory setups, especially with plate splitters or broadband sources. The three lights typically represent:Reflected primary beamTransmitted primary beamSecondary reflection or wavelength/polarization-specific beam Understanding this behavior is crucial for precise optical alignment, interferometry, and imaging applications, where ghost beams or unintended reflections can affect measurements . Proper coatings, wedging, or using cube splitters can reduce unwanted third beams.