Which property is used to quantify the stiffness of a soft contact lens material?

• A. Refractive index
• B. Water content
• C. Modulus of elasticity
• D. Surface roughness

Answer: (C)

Stiffness is about how much a material resists deformation under load, and the property that directly quantifies this is the modulus of elasticity. In soft contact lens polymers, Young's modulus describes the stress required to produce a given strain, so a lens with a higher modulus will be stiffer and resist bending more, affecting handling and how the lens sits on the eye. A lower modulus means the lens is softer and more flexible, conforming more easily to the corneal surface. Refractive index is an optical property and tells how light is bent, not how the material resists deformation. Water content relates to hydration and can influence flexibility indirectly but does not measure stiffness itself. Surface roughness concerns the lens’s surface texture, affecting comfort and tear film interaction rather than bulk mechanical resistance.