Microwave oven mica waveguide cover plate for GE JE1860G, part # WB06X10162

Microwave ovens have a waveguide cover made from a thin sheet of mica material that allows the microwave energy to enter the oven cavity, but prevents food splashes from contaminating the waveguide.

Over time, the mica sheet breaks down and eventually needs to be replaced.
The replacement parts are available for many models, but it costs a lot less to buy a sheet of mica and cut it to fit.
Search for "microwave waveguide cover" and you'll find lots of products available.

I have a GE JE