Spectral Transmittance Of Christiansen Filters - Experimental Observations

The spectral transmittance of an inhomogeneous medium consisting of a powder of a transparent solid material immersed in a different material, such as a liquid, depends on the difference of refractive indices of both materials. If these coincide, the spectral transmittance is maximum. With increasing absolute difference of the refractive indices, the spectral transmittance of a direct beam of electromagnetic radiation decreases. This is the origin of a bell-shaped transmittance curve of the inhomogeneous medium. A plane-parallel layer of such a medium is called the Christiansen filter, in honour of the author who described this effect more than 100 years ago.