Glass is a refractive material. To get real looking glass, you'd best plug a Fresnel node into the Refraction input of the root material node. Refraction index of glass is about 1.45.

There's a bit of a problem though. Refraction in Poser requires that the refractive object does NOT cast a shadow, and that it is NOT transparent. The frame of your pair of glasses won't be transparent, but you probably want it to cast shadows!

The solution is to make the glasses a two-part prop: the frame (parented to the head) and the lenses (parented to the frame). 

Also, for refraction to work, Poser requires a minimum of 2 raytrace bounces. Since the light rays enter the glass from one side, leave it on the other side, hit the face and bounce back through the lens again, you'll need 4 raytrace bounces.

I hope the glass portion is modeled as a double sided lens, and not as a simple one-sided flat shape. If it is, you can get "real" reflections/refractions from the lens. If not, you can make it a double sided lens shape by creating an extra prop from the glass material using the Grouping Tool. Flip the normals of the spawned glass, it'll be the "inside" of the lens.
Use magnets to shape the two glass parts into lens shape. Then it should work.

It'll take a lot of test renders to get things right. But you'll be basing your work on physical reality, which (hopefully) can lead to a high degree of realism.

Good luck!

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