Certified Paraoptometric (CPO) Practice Exam 2025

Which type of lens is made from materials that are oxygen-permeable but do not absorb water?

1. Soft

2. Rigid

3. Hybrid

4. Bifocal

The correct answer is: Rigid

The correct choice pertains to rigid lenses, specifically rigid gas permeable (RGP) lenses. These lenses are designed from materials that allow oxygen to pass through to the cornea, which is essential for maintaining corneal health, yet they do not absorb water like soft contact lenses. This is a significant advantage because the lack of water absorption helps maintain the lens stability and clarity over time, making them suitable for individuals with various vision correction needs. The increased oxygen permeability are particularly beneficial for extended wear, reducing the risks associated with hypoxia, such as eye infections and discomfort. Soft lenses, on the other hand, are typically made from hydrophilic materials that do absorb water and provide comfort, but they may not offer the same level of oxygen transmission as RGP lenses. Hybrid lenses combine the properties of both soft and rigid lenses, incorporating a soft skirt for comfort and a rigid center for vision correction, but they also can retain water in the soft material. Bifocal lenses are primarily defined by their optical design for near and distance vision correction rather than the materials they are made from.