Q. Explain basic working of Interference Filter?

An interference filter, which reflects one of more spectral lines and transmits others, while maintaining a nearly zero coefficient of absorption for all wavelengths of interest. In an optical filter, a semitransparent metal film is deposited on a plate of glass. Then, it is coated with a thin layer of some dielectric material like MgF₂, followed by another coating of a thin film of metal. Finally, another plate of glass is kept over the films for mechanical protection. Interference filters have a band pass of 100-150A and peak transmittance of 40-6-%. When a beam of light is incident normally on an interference filter, multiple reflections takes places between the two surface (Layers).

If a thin transparent spacer is placed between two semi reflective coatings, multiple reflections and interference can be used to select a narrow frequency band, producing an interference filter. It the spacer is half wavelength for the desired wavelength, then other will be eliminated by destructive interference. If the back layer is totally reflective, then the arrangement is called a dichotic mirror, reflecting only the selected wavelength.