Calcite

This is a calcite crystal in a mounting medium of 1.664 refractive refractive index index medium with the linear polarizing filter aligned with the omega direction (bisecting the acute angle). The colors here indicate a refractive index match at a wavelength of approximately 560 nm. The crystal has higher refractive indices for wavelength in the red and the liquid has higher refractive indices in the green and blue end of the spectrum. These are the Becke' lines seen as the particle is defocused by moving further away from the objective.