Cutaway render of the mesh corneas:

The optics morph affects the shape: Default is parallel, dialed into positives it increases thickness and makes it a convex lens, dialed into negatives it makes it concave. It's more or less parallel but if you want to play with the optics - which dramatically affects the refraction - then go for it. My first prototype cornea was a slight lens shape and when I rendered it the effect was way too pronounced so I made these adjustable.

According to https://pixelandpoly.com/ior.html the index of refraction should be around 1.38