This is a transparent plate with two semi-reflecting surfaces. Once light has entered, it is reflected back and forth between the two surfaces and undergoes interference.

Standing waves are therefore formed in the space between the two surfaces, as in a resonant cavity; frequencies that are not in resonance undergo destructive interference and therefore cancel each other out, while those in resonance undergo constructive interference and are therefore transmitted.

The net result is that the device lets through only light of a very precise frequency (within a range that depends on the efficiency of the reflections inside the plate).

The wavelength of the transmitted light depends on the thickness of the plate, on the material it is made of (that is, on its refractive index) and on the angle at which the light strikes it.