IR Filters can sometimes see through materials that are opaque or difficult to see through.  This effect is also quite useful for factory automation and computer control.

The following pictures are un-retouched actual photographs.  

Equipment Used:  IR-Enabled Sony DSC-S30 Digital Camera & X-Nite780nm filter

Lighting: Incandescent

                    Normal View                                                    Infrared View

Not all sunglasses are IR transparent.  Below is an example of some Bosch & Lomb sunglasses that are not particularly IR transparent.  The most transparent sunglasses are the inexpensive plastic sorts.