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Interference Filter

An interference filter is based on the principle of Fabry-Perot interferometer. It consists of an optical system that will transmit nearly a monochromatic beam of light (covering a small range of 50 Å).

An interference filter consists of a thin transparent dielectric e.g. Magnesium fluoride. These are two glass plates on whose surfaces semi-transparent silver films are deposited by evaporation method. The dielectric is placed between the two glass plates.

When a beam of light is incident normally on the filter, multiple reflection take place within the film. The interference maxima for the transmitted beam will be governed by

Here is the refractive index of the dielectric t is its thickness, and n is a whole number. If n will be equal to 2. For the value of n = 1, the maximum occurs for a wavelength of 2λ. Here λ and 2λ represent a wide separation in the visible region.

In the case of an interference filter, when the thickness of the dielectric is reduced, the transmitted wavelengths are more widely spaced. For an optical thickness of the dielectric film of 5000 Å, the transmitted wavelength for n = 1, 2, 3 etc. are 10000 Å, 5000 Å, 3333 Å. These three wavelengths are widely spaced. Only 5000 Å is in the visible region. If there are two maxima in the visible region one of them can be eliminated by using a coloured glass filter. This may be the protecting glass of the dielectric itself.

Interference filters are better as compared to the coloured glass filters because in the case of interference filters light is not absorbed and hence there is no overheating. Interference filters are used in spectroscopic work for studying the spectra in a narrow range of wavelengths.