High-index = Lenses that are thinner & lighter in weight because the material is denser.
Standard plastic (CR-39) is the material of choice with low to moderately strong prescriptions. It is relatively inexpensive, lighter than glass and scratch resistant. From moderately strong to severe prescriptions different “high-index” materials are called for. The term “high-index” refers to any lens material with an index of refraction greater than the 1.49 of CR-39.
All lens materials being denser than air have higher refractive indices ranging from the 1.49 of CR-39 through 1.60 through 1.67 to finally 1.74, corresponding to progressively denser and thinner high-index lenses. 1.60 lenses are about 30-35% thinner than CR-39, 1.67 lenses are about 40% thinner, and 1.74 are about 50% thinner than CR-39.