Certified Paraoptometric (CPO) Practice Exam 2025

Which type of lens is recognized as the hardest and most scratch-resistant?

1. Polycarbonate

2. CR-39

3. High-Index lenses

4. Glass

The correct answer is: Glass

The hardest and most scratch-resistant type of lens is glass. Glass lenses offer superior optical clarity and have a much higher degree of hardness compared to other materials. This hardness makes them more resistant to scratches, providing a durable option for those who prioritize lens longevity. Glass lenses are often preferred in situations where robustness is a key requirement, such as in safety eyewear. While other materials like polycarbonate and high-index lenses offer benefits such as being lightweight and impact-resistant, they do not match glass in terms of scratch resistance. Polycarbonate is known for its excellent impact resistance, making it a popular choice for children and active lifestyles, but it is more prone to scratches compared to glass. Similarly, high-index lenses, which are designed to be thinner and lighter, still fall short of the scratch resistance that glass lenses provide. Therefore, glass is recognized for its durability and scratch-resistant qualities.