Question

The fibre which is having negative birefringence value is

• A. Cotton
• B. Nylon
• C. Silk
• D. Acrylic

Hint:

Cotton is the most common fiber with negative birefringence. Most other fibers, especially synthetics, have positive birefringence.

Answer 1

The Correct Option is D

Birefringence is the difference between refractive indices in different directions of a material. Positive birefringence value indicates that the refractive index along the fiber axis is greater than the refractive index perpendicular to the fiber axis. Negative birefringence value signifies the opposite: the refractive index along the fiber axis is less than that perpendicular to the axis.

Most natural fibers like cotton and silk, and some synthetics like nylon, exhibit positive birefringence. This is because their molecular chains are oriented along the fiber axis, leading to a higher refractive index in that direction.

Acrylic, however, is different. Its structure, often with bulky side groups projecting outwards from the main chain, leads to a higher refractive index perpendicular to the fiber axis, resulting in negative birefringence. This distinctive optical property is one way to differentiate acrylic from other fibers.