Birefringence

Birefringence

Light passing through a mineral is resolved into two permitted vibration directions. In an anisotropic mineral light travels at different velocities along the two vibration directions. The ray travelling in the slow vibration direction will be retarded compared with the ray travelling in the fast vibration direction. On leaving the mineral grain the two rays will interfere, producing interference colours.

When the slow ray exits the mineral grain the fast ray will already have exited the grain and travelled an additional distance know as the optical path difference, or retardation (Δ). This distance is proportional to the thickness of the mineral grain (t) and the difference in the refractive indices of the two vibration directions (n₁-n₂):

Δ = t(n₁-n₂)

The birefringence is the difference in the refractive indices of the two vibration directions (n₁-n₂).

When rotated under cross-polarised light anisotropic minerals display changing interference colours called birefringencecolours. The birefringence (n₁-n₂) of the mineral can be determined from these colours using a Michel-Levy chart.