Xtreme Fox

Glasses Beauties - part 110; addendum 2

It is rather easy to answer his question:

His old pair of glasses had obviously lenses made of real glass; maybe even of a type of glass with a specially high refraction index. For the new pair of glasses he obviously got lenses made of a polymer (plastic). This kind materials does have a much weaker refraction index, so the curvature of the inner surface of the lens must be much stronger to achieve the same optical refraction as with a lens made of real glass. The result of this is that the edges of lenses made of "plastic" are much thicker than those of lenses made of real glass for the same prescription.

In case of guys like him and me too, who we are very strongly nearsighted, it is always a dilemma to decide if it is better to get the much thinner lenses of glass for the next pair of glasses which on the other hand side are heavier than those made of "plastic" or if it is better to get those platic lenses which are much lighter, but also much thicker at their edges.