Minus lens center thickness around 2.2 mm could indicate which material?

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

Answer: (C)

The key idea is how lens material affects center thickness in minus (concave) lenses. For a given prescription and frame, higher-index materials let you achieve the same optical power with less curvature, which tends to produce thinner centers. Lower-index materials require more material to reach the same power, leading to thicker center sections.

In practice, glass lenses, being denser and often with a lower practical index compared to the latest plastics, tend to end up with thicker centers for a given minus prescription and frame, sometimes around a couple of millimeters. A center thickness near 2.2 mm is therefore more consistent with glass than with the plastic options listed, which can be made thinner thanks to their higher indices.

So a center thickness around 2.2 mm could indicate glass.