Which lens generally has a lower index of refraction compared to crown glass?

CR-39 lens is a type of plastic lens commonly used in eyewear, known for its lightweight properties and decent optical quality. Its index of refraction typically hovers around 1.50, which is indeed lower than that of crown glass, which usually has an index of refraction around 1.52.

This lower index of refraction allows CR-39 lenses to be thinner and lighter than some other lens materials, making them more comfortable for daily wear in many prescriptions. The use of CR-39 as a lens material has become popular due to these traits as well as its affordability and durability compared to glass options.

In contrast, polycarbonate lenses typically have a higher index of refraction (about 1.586), making them thicker than CR-39. Sclera lenses, primarily used in contact lenses for vision correction, also have varying indices that do not typically reflect a lower index of refraction than crown glass. Flint lenses, known for their high optical quality, usually have an even higher index than both crown glass and CR-39.