Certified Paraoptometric (CPO) Practice Exam 2025

Which type of lens typically has the hardest surface and is also the heaviest?

1. Plastic lens

2. Polycarbonate lens

3. Glass lens

4. Acrylic lens

The correct answer is: Glass lens

The correct answer is glass lens, as glass lenses are known for having superior optical clarity and a very hard surface. The manufacturing process for glass allows it to be polished to a high degree, making its surface resistant to scratches compared to plastic and acrylic alternatives. Additionally, glass lenses tend to be heavier than their plastic and polycarbonate counterparts due to the density of the material used. While plastic and acrylic lenses are lighter and more shatter-resistant, they do not possess the same hardness and surface durability as glass. Polycarbonate lenses, while lightweight and highly impact-resistant, are not as hard as glass. Therefore, glass lenses stand out for both their hardness and weight, making them the correct choice in this context.