Which lens material has the lowest index of refraction?

Which lens material has the lowest index of refraction?

• A. Ophthalmic crown glass
• B. CR-39
• C. Polycarbonate
• D. Trivex

Answer: (B)

Refractive index tells you how much a material slows and bends light entering from air. Among common ophthalmic lens materials, CR-39 has the lowest index (about 1.498), while crown glass is around 1.523, Trivex about 1.532, and polycarbonate around 1.586. So CR-39 bends light the least for a given surface shape, which is why it’s the lowest-index option in this set. Because lower index material bends light less, a lens made from it would generally be thicker for a strong prescription than a lens made from a higher-index material; higher-index plastics are used to keep lenses thinner at higher powers.

Refractive index tells you how much a material slows and bends light entering from air. Among common ophthalmic lens materials, CR-39 has the lowest index (about 1.498), while crown glass is around 1.523, Trivex about 1.532, and polycarbonate around 1.586. So CR-39 bends light the least for a given surface shape, which is why it’s the lowest-index option in this set. Because lower index material bends light less, a lens made from it would generally be thicker for a strong prescription than a lens made from a higher-index material; higher-index plastics are used to keep lenses thinner at higher powers.