OPTICAL PROPERTIES

All the common textile fibres are anisotropic; that is to say there is a directional nature about many of their properties. Where on the other hand, a material has the same properties when measured in an direction, it is termed as isotropic. 

The directional nature of the properties of many textile fibres may be seen in considering the dimensions, for fibres are longer than they are wide; fibres are stronger in the direction of long axis then across their width. When fibres swell in water or other liquid there is a much greater increases in width then in length. Anisotropic character indicates the building up on a crystalline pattern. The velocity of light within the fibre depends on the direction in which it is measured is an instance of anisotropy.

The refractive index of all crystals varies with the plane of vibration of the incident light i.e. they are optically anisotropic, the property varying according to the direction in which it is measured. [This doable refraction or birefringence is positive if the refractive index is greater in the axial direction, and negative if it is grater in the transverse direction. The phenomenon is due to the different atomic spacing in the different directions of the crystal lattice allowing the light to pass with different velocities.]