Microwaves are designed with windows to allow users to see inside and monitor the cooking process without having to open the door. The windows typically consist of a metal mesh or a special type of glass that allows visible light to pass through while blocking most of the microwave radiation.
The reason why looking into or standing in front of an operating microwave is cautioned against is due to the potential exposure to microwave radiation. Microwaves use electromagnetic waves at a frequency of around 2.45 GHz to cook food. These waves are absorbed by water, fats, and sugars in the food, causing them to heat up and cook the food quickly and efficiently.
If the door of a microwave is damaged, has a faulty seal, or is not closed properly, there is a risk of microwave radiation leakage. However, microwave ovens are designed and manufactured to meet strict safety standards to prevent such leaks under normal usage. The mesh or glass window on the door is specifically designed to contain the microwaves within the oven during operation.
As long as the microwave is functioning correctly and the door is intact and undamaged, standing in front of a microwave or looking through the window while it's operating is generally considered safe. The microwaves are contained within the oven, and the window does not allow harmful radiation to pass through.
However, it's always essential to follow the manufacturer's instructions and safety guidelines for your specific microwave model. Additionally, if you notice any damage to the door or suspect that the microwave is not functioning correctly, it's best to stop using it and have it inspected or repaired by a qualified technician.