Dichroic Crystals

Some crystals are inherently dichroic. This is due to the particular arrangement of the atoms in the crystal lattice. For materials that exhibit dichroism, there is a specific direction within the crystal known as the principal or optic axis.

The component of the electric field vector that is perpendicular to the optic axis is strongly absorbed. At this point you might be wondering to yourself why the differential absorption of orthogonal polarization states is known as dichroism, which seems to suggest color dependence.

The phrase comes from the fact that, for some crystals exhibiting dichroism, there is residual absorption of the light that should have been transmitted. To make matters worse, the absorption of the transmitted beam is also wavelength dependent. Therefore, a crystal of only a few millimeters thick might appear green (all other wavelengths were absorbed) when viewed at one angle, and then completely dark when viewed at another angle, hence -- dichroic, meaning two colors).

Follow the steps below to explore dichroism, then answer the questions for this activity:

1. Go to the EMANIM website
2. Click the checkbox to 'turn on' Wave 2
3. Click the checkbox to 'turn on' Wave 1 + Wave 2
4. Increase the amplitude of Wave 2 to 5 (this now matches the amplitude of Wave 1)
5. Scroll down and click the checkbox to 'turn off' Components
6. Change the phase difference for Wave 2 to 90 (this is in degrees) -- observe the superposition of waves
7. Click the checkbox to 'turn on' a Material -- note that an orange parallelepiped appears to indicate the material volume
8. Turn up the extinction coefficient for Wave 2 to produce a GIF similar to the one below