The Mesh Shield

The metal mesh in a microwave door is a Faraday cage. Its holes are small enough to trap microwaves but large enough for visible light to pass through.

The door seal does the rest of the job — it reflects stray waves back inside before they reach the gap around the door. Thanks to the mesh and the seal working together, modern microwaves leak almost zero radiation. The most dangerous thing about yours is burning your tongue on unevenly heated food.