Why do microwave doors have metal mesh?
Short answer
The metal mesh helps keep the microwaves used for heating inside the oven while still letting visible light pass through so we can see the food.
How it works
Microwave ovens use electromagnetic waves with wavelengths much longer than visible light. The tiny holes in the door mesh are small compared with the microwave wavelength, so microwave energy is strongly blocked from passing through them.
Visible light has much shorter wavelengths, so it can pass through the openings and reach our eyes. The surrounding metal door and its seal also help contain the electromagnetic energy inside the cooking cavity.
Key point
Do not use a microwave oven if its mesh, door hinges, latch, or seals are damaged, or if the door does not close firmly. Whether a particular metal container is safe depends on the appliance, so follow the manual rather than assuming the mesh makes it safe.