Which statement correctly lists advantages of glass over CR-39 lenses?

Which statement correctly lists advantages of glass over CR-39 lenses?

• A. Does not scratch as easily
• B. They are thinner because of the higher index of refraction
• C. Does not warp as easily
• D. All of the above

Answer: (D)

The main idea is how lens material properties affect performance: scratch resistance, potential for thinner lenses due to refractive index, and dimensional stability. Glass has a harder surface than CR-39, so it resists scratches much more effectively, helping maintain clarity over time. It also typically has a higher refractive index than CR-39, meaning that for the same prescription you can design lenses that are thinner, especially in higher powers. Additionally, glass is very rigid and dimensionally stable, so it doesn’t warp as easily as a softer plastic like CR-39. Because each of these statements is true, the option that includes all of them best captures the advantages of glass over CR-39. Keep in mind that glass is heavier and more prone to breakage, which are practical trade-offs.

The main idea is how lens material properties affect performance: scratch resistance, potential for thinner lenses due to refractive index, and dimensional stability. Glass has a harder surface than CR-39, so it resists scratches much more effectively, helping maintain clarity over time. It also typically has a higher refractive index than CR-39, meaning that for the same prescription you can design lenses that are thinner, especially in higher powers. Additionally, glass is very rigid and dimensionally stable, so it doesn’t warp as easily as a softer plastic like CR-39. Because each of these statements is true, the option that includes all of them best captures the advantages of glass over CR-39. Keep in mind that glass is heavier and more prone to breakage, which are practical trade-offs.