How does soft contact lens rigidity change as lens thickness increases?

How does soft contact lens rigidity change as lens thickness increases?

• A. Increases
• B. Decreases
• C. Remains constant
• D. Unrelated

Answer: (A)

Rigidity here means bending stiffness. For a soft contact lens made of the same material, bending stiffness grows as thickness increases, because there’s more material to resist bending. The bending (flexural) rigidity of a thin plate is proportional to E t^3 (with a small correction for Poisson’s ratio), where E is the material’s Young’s modulus and t is thickness. Practically, this means doubling the thickness makes the lens much harder to bend (roughly eight times stiffer). So thicker soft lenses feel stiffer on the eye, not more flexible.

Rigidity here means bending stiffness. For a soft contact lens made of the same material, bending stiffness grows as thickness increases, because there’s more material to resist bending. The bending (flexural) rigidity of a thin plate is proportional to E t^3 (with a small correction for Poisson’s ratio), where E is the material’s Young’s modulus and t is thickness. Practically, this means doubling the thickness makes the lens much harder to bend (roughly eight times stiffer). So thicker soft lenses feel stiffer on the eye, not more flexible.