The grid on the microwave oven window is commonly known as the "microwave screen" or "mesh." Its purpose is to prevent microwave radiation from escaping the oven while still allowing you to see the food cooking inside.
Microwaves are a form of electromagnetic radiation, specifically in the microwave frequency range. These waves have relatively long wavelengths and can pass through certain materials, like glass and plastic, but they are blocked by conductive materials like metal.
The microwave screen on the oven door is made of a fine mesh or grid of conductive metal, typically a combination of aluminum and other materials. When microwaves hit this metal grid, they induce small electric currents in the metal. These induced currents, in turn, generate electromagnetic fields that oppose and cancel out the incoming microwaves.
The holes in the metal mesh are designed to be much smaller than the wavelength of the microwaves, which prevents the microwaves from passing through. The holes are also large enough to allow visible light to pass through, so you can see inside the microwave while it is operating.
In summary, the metal mesh acts as a barrier, reflecting and blocking the microwaves from escaping, while still allowing visible light to pass through, allowing you to monitor the cooking process safely. It's important to ensure that the microwave screen is in good condition and not damaged, as any tears or holes could potentially allow microwave radiation to leak out, which would be hazardous.