Lipids in crystalline form are described as which combination of optical properties?

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

Answer: (A)

Lipids in crystalline form have a dual optical behavior: they look isotropic under ordinary, unpolarized light, but they become birefringent when viewed with polarized light. Isotropic means no change in light’s polarization in normal illumination, so the crystals appear uniform. When polarized light is used, the ordered crystal structure causes double refraction, splitting light into two rays and producing birefringence. That makes the description isotropic and birefringent the best fit: isotropic in plain light, birefringent under polarized light. The other options don’t capture this dual behavior—anisotropic would imply birefringence in all conditions, isotropic only would mean no birefringence under polarized light, and birefringent only would imply birefringence even in ordinary light.