Lens materials come in refractive indices from 1.49 up to 1.9. In general terms, the higher the refractive index, the thinner the lens will be. Plastic lenses are available in 1.49, 1.56, 1.6, 1.67, 1.71 and 1.74 indices. Glass is available in 1.523, 1.6, 1.7, 1.8, and 1.9 so for short sighted people (minus prescriptions) glass is the thinnest option. Glass however is roughly twice the weight of plastic and can cause serious eye injury if you are at risk of breaking the lenses (for example doing DIY or playing sports). Except for the very thinnest lenses where there is no plastic equivalent, glass is rarely dispensed these days.