Certified Paraoptometric (CPO) Practice Exam 2025

What is a notable characteristic of CR-39 lenses compared to glass lenses?

1. More scratch-resistant

2. Thicker than glass

3. Less likely to break

4. Significantly lighter than glass

The correct answer is: Thicker than glass

The notable characteristic of CR-39 lenses is that they are significantly lighter than glass lenses. CR-39, which is a type of plastic optical lens material, was specifically designed to provide a lightweight alternative to traditional glass lenses, making them more comfortable for prolonged wear. This lightweight nature is particularly beneficial for prescriptions that require thicker lenses, as they can help reduce the overall weight of the eyewear. In contrast, while CR-39 lenses may be more impact-resistant and less likely to shatter compared to glass lenses, they do not boast the same level of scratch resistance as some types of glass lenses, which are often treated for enhanced durability. Additionally, the thickness of CR-39 lenses can vary based on prescription requirements and is not universally thicker than glass. Therefore, the significant lightness of CR-39 compared to glass is the defining characteristic that sets them apart.