Microwave ovens are designed with a perforated metal screen on the door and the window to prevent microwave energy from escaping while still allowing you to observe the cooking process. This screen is known as a "Faraday cage" or "Faraday shield."
The Faraday cage is named after the English scientist Michael Faraday, who discovered the principle behind it in the early 19th century. It works based on the principles of electromagnetic shielding. When microwave energy is produced inside the oven's cooking chamber, it consists of electromagnetic waves in the microwave frequency range. These waves are essentially electromagnetic radiation, similar to radio waves but at a much higher frequency.
The holes in the metal screen are small in comparison to the wavelength of the microwaves (which are typically around 12.2 cm or 4.8 inches for household microwave ovens). When the size of the holes is significantly smaller than the wavelength, the electromagnetic waves cannot easily pass through the gaps. Instead, they encounter the metal grid, which reflects and absorbs the microwave energy, preventing it from escaping the oven and potentially interfering with other electronic devices or posing a safety hazard.
The Faraday cage effect can also be observed with other electronic devices, such as smartphone cases that contain metallic elements to block electromagnetic interference (EMI).
In summary, the holes in the screen of a microwave oven are small enough to block the microwaves from escaping while still allowing you to see inside the oven during the cooking process. This design ensures the safe and efficient containment of the microwave energy for cooking food while preventing potential interference with other electronic devices in the surrounding area.