Lipids in crystalline form are described as which of the following?

• A. Isotropic and birefringent
• B. Anisotropic
• C. Isotropic
• D. Birefringent

Answer: (A)

Birefringence is a property that shows up when a material has directional order, meaning its refractive index differs along different axes. Crystalline lipids organize into ordered, anisotropic structures, so light traveling through them splits into two rays with different speeds. Under crossed polarizers this creates interference colors—the telltale sign of birefringence. If a substance were isotropic, its refractive index would be the same in all directions and it wouldn’t exhibit birefringence. So crystalline lipids are birefringent because of their directional molecular arrangement, not isotropic.