Microwave ovens are designed with a specific type of door, typically made of a transparent material like glass or plastic, that allows you to see inside while preventing microwaves from escaping. The door of a microwave oven contains a metal mesh or grid called a "Faraday shield" that serves as a barrier to the microwaves.
The holes in the metal mesh of the door are smaller than the wavelength of the microwaves (around 12.2 centimeters or 4.8 inches for household microwaves), which effectively blocks the microwaves from passing through. The size of the holes in the mesh is carefully chosen to prevent microwaves from escaping while still allowing visible light to pass through, which is why you can see inside the oven.
When the microwave oven is operating, the microwaves generated by the magnetron (the microwave emitter) bounce around inside the oven's metal cavity, where they are absorbed by the food or liquid. The metal walls of the oven reflect and contain the microwaves, ensuring they stay inside the oven.
The door with the metal mesh allows you to view the food or liquid inside, but the small holes in the mesh prevent the microwaves from passing through. This design feature ensures that the microwaves are contained within the oven, so they don't escape and cause potential harm to the user or interfere with other electronic devices.
It's important to note that the metal mesh on the door may cause some minimal leakage of microwaves, but it is designed to be within safe limits specified by regulatory standards. As long as the microwave oven is in good working condition and the door is not damaged or compromised, it effectively contains the microwaves, preventing them from cooking beyond the plastic door.