A branch of optics in which the wave nature of molecules is studied. To see wave behaviour in molecules it helps to use very low temperatures; in this way the velocities of the molecules are reduced, thus increasing their de Broglie wavelengths. Even large molecules such as cluster molecules have exhibited wave behaviour. This type of optics has great promise for investigating the fundamentals of quantum mechanics. In addition, it has been possible to produce a version of Poisson’s spot for quantum-mechanical matter waves using molecular matter wave optics.