What are the various Eyeglass Lens materials?

Spectacle lenses are made of two main types of materials - plastic or glass. Plastic lenses are often CR39 or polycarbonate. Both main materials come in a variety of refractive indexes, designed to minimise the thickness. HiIndex lens materials are commonly 1.56, 1.60 and 1.67, as compared to 1.523 for common crown glass. The refractive index of plastic lenses is capped at around 1.74, whereas glass lenses range up to an index of 1.9, making them potentially thinner. However, hi-index glass lenses are very brittle and not suitable for many activities which carry a risk of lens impact. Ultimately, the lens choice is very much a user application. Hi Index will provide a thinner edge and better cosmetic appearance but still weigh in heavy. Comparatively, plastic lenses are lighter but in high minus prescriptions will have a significantly thicker lens edge. Furthermore the size of the frame will influence the overall edge thickness since a larger eyesize means a larger lens.  more