What is the purpose of the grid on the glass door of the microwave oven?
The metal grid reflects microwaves and keeps them inside the oven. Electromagnetic waves are unable to pass through holes in conducting materials if those holes are significantly smaller than their wavelengths. The wavelengths of visible light are very short, so light has no trouble passing through the holes in this grid. But the microwaves used in the oven have wavelengths of about 12.4 cm and are unable to propagate through the grid. Thus you can see the food cook while the microwaves are trapped inside the oven.