The holes on the inside of microwave oven doors serve a crucial safety function. They are designed to allow you to see into the microwave while it's operating, without exposing you to harmful microwave radiation.
Microwave ovens use a type of electromagnetic radiation called microwaves to cook or heat food. These microwaves are contained within the oven's cooking chamber by a metal mesh screen in the door. The holes in the door are part of this metal mesh.
Microwaves have a longer wavelength than visible light, so they can't be seen with the naked eye. However, they can still pass through the small holes in the metal mesh of the door, allowing you to see the food inside. The size and spacing of these holes are carefully designed to prevent microwaves from escaping in a dangerous amount.
Without these holes and the metal mesh, microwaves could leak out, potentially causing harm to anyone nearby and interfering with other electronic devices. The design of the microwave oven door is tightly regulated and tested to ensure that it meets safety standards and keeps users protected from excessive microwave radiation exposure while allowing them to observe the cooking process safely.