birefringence: one construct, many systems

In crystal optics, birefringence is the splitting of a light beam into two rays that travel at different speeds and along different paths through an optically anisotropic material, the effect first famously seen as the double image through a crystal of Iceland spar. Brewster's 1815 Experiments on the depolarisation of light as exhibited by various mineral, animal, and vegetable bodies surveyed the phenomenon across many materials and tied it to the general principles of polarisation. The referent is a material's difference in refractive index with polarisation direction, measured by that index difference and the retardation it produces, and the same construct recurs wherever matter becomes optically anisotropic, under stress or in living tissue.[1]

Unit

The difference in refractive index between the two polarisation directions

Recovery means

Not applicable

Measured by

Gauged by the refractive-index difference and the optical retardation, seen as interference colours under a polarising microscope

In experimental mechanics, a normally isotropic transparent material becomes birefringent when it is stressed, with the induced index difference proportional to the stress, so that a loaded model viewed in polarised light reveals its stress field as a pattern of coloured fringes. Dally and Riley's Experimental stress analysis sets out this photoelastic method and how the fringes are read to map stress. It is the same double-refraction construct as in a crystal, here produced by mechanical strain rather than fixed crystal structure, and measured through the same retardation and fringe orders.[2]

Unit

The stress-induced index difference, read as photoelastic fringe order

Recovery means

Not applicable

Measured by

Read from photoelastic fringe orders in a polariscope, converted to stress through the material's stress-optic coefficient

In biology, ordered molecular structures such as collagen, muscle filaments, starch grains, and the mitotic spindle are birefringent, because their aligned molecules make the refractive index depend on polarisation, which is why such structures shine against a dark field under a polarising microscope. Oldenbourg's 1996 A new view on polarization microscopy introduced an imaging method that maps this birefringence quantitatively in living cells without staining. It is the same optical-anisotropy construct as in minerals and stressed solids, here arising from biological molecular order and used to reveal structure non-destructively.[3]

Unit

The retardation from aligned molecular structure in tissue

Recovery means

Not applicable

Measured by

Imaged as optical retardation and slow-axis orientation by quantitative polarised-light microscopy