When light passes into a material medium, there is an interaction between the visible light photons and the electrons of the substance. This interaction will decrease the speed of light in this medium and change the direction of light propagation. In this image, we see crystals of calcite, with a molecular structure that does not have the same refractive properties in all directions. The incident light penetrating the crystals will have a different speed along its axis of polarization. Calcite therefore has two different refractive indices; it is birefringent. The light that emerges will have followed two different trajectories and each trajectory will be associated with a polarization perpendicular to the other. This can be observed by the splitting of the gridlines seen in the crystals and the observation of a single grid when placing a polarizing filter (black circle) on the same crystals.