Index Of Refraction
If you have a high prescription (give or take +/- 3 diopters) you've probubly been told that you should go with a Hi-Index lens material in order to make the lenses thinner. So what exactly is Hi-Index?
All substances that allow light to enter and leave can be assigned an index of refraction (IOR). The number essencially defines how that substance bends (refracts) light.
Here are some optical lens materials with their IOR:
CR-39 - 1.498
Trivex - 1.54
Polycarbonate - 1.586
Hi-Index - 1.67
Hi-Index - 1.74
So let's say someone has a -4.00 prescription; depending on the size of the frame they select, using a low index material like CR-39 can lead to undesired thickness. A higher index material will provide the same correction but will produce less thickness.
So this is generally why opticians will suggest going with a Hi-Index material when a patient has a high prescription.
Now, it may seem relative, but a Hi-Index material can produce a thinner lens but that doesn't make it lighter weight. In fact, the density of Hi-Index lenses often leads to a heavier weight over materials such as Polycarbonate and Trivex.
There have been plenty of situations where patients have preferred a lightweight lens over a thinner lens.