Increasing center thickness of a GP lens will cause the Dk to:

• A. Higher
• B. No change
• C. Depends on design
• D. Lower

Answer: (D)

The key idea is that oxygen transmission through a lens depends on both the material’s permeability and the path length the oxygen must travel. Dk is the material’s intrinsic permeability, a fixed property, while the effect of thickness is captured by Dk/t (oxygen transmissibility). Increasing center thickness lengthens the diffusion path, so for the same material Dk/t decreases. In other words, thicker center yields less oxygen reaching the cornea, making the effective transmissibility lower. If you want more oxygen to the eye, you’d use a material with a higher Dk or reduce the lens thickness; thickness directly lowers the available oxygen, even though the material’s Dk itself doesn’t change.