Thin film effects are best understood when considering the optical radiation as a wave rather than a particle. Thin film effects occur when thin layers of alternating high and low index materials are in the path of radiation. Depending on the indices of refraction of the layers and their thickness, waves traveling in the incident direction will either pass through the film, or be cancelled by antiphase waves traveling away from the incident direction. It is not uncommon for an optical filter to apply both principles, where a thin film complex is coated on an absorptive substrate. These filters are also commonly called interference filters.