Collections of solid particles that are transparent to
electromagnetic radiation of a particular
wavelength and are slightly larger than the
wavelength of the radiation transmit poorly due to
reflection and scattering from the interfaces. The transmission increases markedly at wavenumbers at which the refractive index of the particles is close to that of the surrounding medium.
Note:
The Christiansen effect frequently causes strong absorption bands from coarsely ground particles to be distorted due to the change in refractive index associated with the anomalous dispersion of the refractive index through the absorption region.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 740 (https://doi.org/10.1515/pac-2019-0203)