Minus lens center thickness between 1 and 2 mm could indicate which material?

• A. CR-39
• B. Glass
• C. Polycarbonate, Trivex, & some high index plastics
• D. Basic impact safety lenses.

Answer: (C)

Minus lenses are thinner at the center and thicker at the edges. The amount of bending a lens provides for a given prescription depends on the material’s refractive index: higher index means more bending per millimeter, so the center can be made much thinner for the same minus power. That’s why having a center thickness around 1–2 mm points to materials with higher indices—polycarbonate, Trivex, and many high-index plastics. Lower-index materials like CR-39 and traditional glass require more thickness to achieve the same corrective effect, so their centers tend to be thicker. Basic impact safety lenses are often polycarbonate, but the thinning cue specifically aligns with higher-index materials.