How does glass create an electric field?
In general, glass front panels are near optimal because they conduct electric fields almost twice as easily as plastic panels.


This means is Glass (\$\epsilon _R =4.7\$) has twice the relative permittivity of plastic ( \$\epsilon _R =2.3\$ ) in the materials preferred here. This applies to both mutual and direct capacitance.

What is free space permittivity ? 
  - about 9 picofarads per meter ( ~ 9 pF /m means means 9 pF  would be the capacitance of two hands with a gap of \$1/9^{th}\$ of the hand width.
Why it is relative? all insulators are dielectric but glass is still twice as much as plastic and more than 4 times air or a vacuum.


  ultra-thin glass panels as retransmission effects can occur   


This means a partial discharge (PD) can occur and the charge buildup can create false sensing, which also may damage cells from the E field breakdown as breakdown threshold for example  >10kV/mm or 10V/um is approached with ultrathin glass &lt; 1mm