Minus lens center thickness between 1.9 and 2.2 mm could indicate which material?

• A. Trivex
• B. Polycarbonate
• C. Glass
• D. CR-39

Answer: (D)

Center thickness in minus lenses is largely dictated by how much material is needed to achieve the given minus power, which depends on the lens material’s refractive index. A lower index means more material is required to bend light the same amount, so the lens ends up thicker in the center. Among common ophthalmic materials, CR-39 has the lowest refractive index (around 1.498). So, for the same frame size and edge thickness, a minus lens with a center thickness in the 1.9–2.2 mm range is most consistent with CR-39. Higher-index materials like polycarbonate, trivex, or higher-index glass would typically yield thinner centers for the same power, hence the observed thickness points to CR-39.