Birefringence in a Calcite Crystal

Iceland spar (calcite) is a material that exhibits birefringence. Due to this phenomenon, light rays with mutually perpendicular polarizations are refracted at different angles. On a screen, we can observe two images of a halogen lamp, each formed by linearly polarized light with perpendicular polarizations.

Experiment according to Birefringence and polarization with calcareous spar.

Experimental setup for birefringence in a calcite crystal. The individual components from left to right are: halogen lamp, adjustable aperture, stage with calcite crystal, lens f = +50 mm, linear polarizer, lens f = –100 mm, screen.

Detail of the calcite crystal.

Two images of the halogen lamp formed on the screen by birefringence in the calcite crystal. Each image is formed by linearly polarized light with mutually perpendicular polarizations.

By rotating the inserted polarizing filter, one image can be removed…

…or the other image. The brightness of the overlap area remains constant during rotation, because according to Malus’s law, the intensities of the individual spots are $I_1 = I_{in} \\cos^2 \\alpha$ and $I_2 = I_{in} \\sin^2 \\alpha$, so $I_1 + I_2 = I_{in}$.

Final side view of the apparatus.